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REESE  LIBRARY 

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BIOLOGY 

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OUTLINES 


OF 


DAIRYBACTERIOLOGY 


A  Concise  Manual  for  the  Use  of 
Students  in  Dairying 


BY 

H.  L.  RUSSELL 

PROFESSOR  OF  BACTERIOLOGY 

University  of  Wisconsin 


UNIVERSITY 


FOURTH  EDITION,  THOROUGHLY  REVISED 


MADISON,  WIS. 

H.    L.    RUSSELL 

1899 


COPYRIGHTED  1894,  1896,  AND  i! 
BY  H.  L.  RUSSELL. 


Tracy,  Gibbs  &  Co.,  Printers,  Madison,  Wis. 


PREFACE  TO  FOURTH  EDITION. 


The  cordial  reception  with  which  these  Outlines  continue 
to  be  received  shows  that  the  effort  to  keep  them  abreast  of 
bacteriological  advance  is  appreciated  by  students  of  dairying. 
An  exposition  of  the  principles  of  bacteriology  as  applied  to 
dairy  problems  is  now  recognized  as  an  essential  part  of  dairy 
science,  for  modern  dairy  practice  is  rapidly  adopting  the 
methods  that  have  been  developed  through  bacteriological 
study. 

Although  a  thoroughly  satisfactory  explanation  has  not  as 
yet  been  given  for  all  the  phenomena  connected  with  milk 
and  its  products,  still  a  better  understanding  of  these  prob- 
lems has  been  rendered  possible  through  the  light  that  has 
been  thrown  upon  these  questions  by  scientific  study  from  a 
bacteriological  point  of  view. 

A  brief  glossary  is  appended. embracing  those  technical  terms 
that  are  necessarily  employed.  My  acknowledgments  are  due 
to  the  following  parties  for  the  loan  of  cuts:  Wisconsin 
Agricultural  Experiment  Station:  Western  Creamery,  San 
Francisco,  Cal.;  Cornish,  Curtis  and  Greene,  Ft.  Atkinson, 
Wis.;  and  A.  H.  Reid  and  Co.,  Philadelphia,  Pa. 

H.  L.  RUSSELL. 
Wisconsin  Dairy  School, 
Madison,  Wis.,  Dec.,  1898. 

82602 


INTRODUCTION. 


Bacteriology  is  the  youngest  member  of  the  sisterhood  of 
those  biological  sciences  that  deal  with  life  in  its  varied  man- 
ifestations and  functions.  While  it  has  a  strictly  scientific 
side,  this  subject  has  a  greater  practical  bearing  than  is  found 
in  many  of  its  sister  sciences.  For  a  long  time  is  was  merely 
a  protege  of  medicine;  even  now,  the  term  bacteria  is  nearly 
always  associated  in  the  minds  of  many  with  some  dread  con- 
tagious disease,  but  with  further  study,  the  effect  of  bacteria 
in  many  other  practical  lines  is  becoming  better  known,  and 
the  circle  of  its  influence  is  steadily  widening. 

Its  methods  have  revolutionized  the  brewing  industries; 
on  the  presence  of  bacteria  depends  the  success  or  failure  of 
many  of  the  industrial  arts,  such  as  butter  and  cheese-making, 
many  of  the  operations  in  tanning,  the  manufacture  of  vine- 
gar, of  wines,  etc.  Modern  agriculture  recognizes  the  effect 
of  germ  life  in  the  various  processes  of  fertilization  by  natural 
manures;  in  the  accumulation  of  nitrogenous  food  in  the  soil 
as  a  result  of  the  action  of  nitrification,  and  in  the  fixation  of 
free  nitrogen  of  the  air  by  members  of  the  clover  family. 

Sometimes  the  bacteria  are  to  us  a  scourge,  but  often,  in- 
deed, they  come  in  the  character  of  a  friend  and  helper. 
More  especially  is  this  true  of  that  class  that  are  associated 
with  dairy  products,  for  in  both  the  butter  and  cheese  indus- 
tries, success  is  only  assured  through  the  activities  of  certain 
favorable  forms. 

The  following  pages  attempt  to  show  this  group  of  organ- 
isms— so  infinitely  small  yet  almost  infinitely  powerful — in 
their  relation  to  the  dairy  and  dairy  products. 

Knowledge  in  dairying,  like  all  other  technical  industries 
has  grown  mainly  out  of  experience.  The  facts  have  been 
learned  by  observation,  but  the  why  of  each  is  frequently 
shrouded  in  mystery. 


vi  Introduction. 

Modern  dairying  is  attempting  to  build  its  more  accurate 
knowledge  upon  a  broader  and  surer  foundation,  and  in  doing 
this  is  seeking  to  ascertain  the  cause  of  well  established  pro- 
cesses. In  this,  bacteriology  is  destined  to  play  an  important 
role.  Indeed,  it  may  be  safely  predicted  that  future  progress 
in  dairying  will,  to  a  large  extent,  depend  upon  bacteriologi- 
cal research.  As  Fleischmann,  the  eminent  German  dairy 
scientist,  says:  "The  gradual  abolition  of  uncertainty  sur- 
rounding dairy  manufacture  is  the  present  important  duty 
which  lies  before  us,  and  its  solution  can  only  be  effected  by 
bacteriology." 


TABLE  OF  CONTENTS. 


PART  1. 
Synopsis  of  Bacteria  in  General. 

Chapter         I.     Structure  and  form 1 

Chapter        II.     Physiology , 5 

Chapter      III.     Methods  of  studying  bacteria 17 

PART  II. 
Bacteria  in  Relation  to  Milk. 

Chapter      IV.     Contamination  of  milk 24 

a.  Infection  on  the  farm 26 

b.  Infection  in  the  factory 46 

Chapter        V.      Milk  fermentations  and  their  treatment 54 

Chapter      VI.     Disease-producing  bacteria  in  milk '.  75 

a.  Infection  direct  from  affected  animal 76 

b.  Infection  subsequent  to  milking 80 

c.  Poison-forming  bacteria 85 

Chapter    VII.     Principles  of  milk  preservation 87 

a.     Chemical  methods 88 

b.     Physical  Methods 90 

Sterilization 94 

Pasteurization    95 

Pasteurizing  apparatus ..." 106 

PART  III. 
Bacteria  in  Relation  to  flilk  Products. 

Chapter  VIII.     Bacteria  in  cream  and  factory  by-products 118 

Chapter      IX.     Bacteria  in  butter-making 127 

a.  Beneficent  action  in  butter 128 

b.  Bacterial  defects  in  butter 145 

Chapter        X.     Bacteria  in  cheese  industry 148 

a.  Bacteria  in  normal  cheese-ripening 150 

b.  Bacteria  in  abnormal  cheese  processes 164 


PART  I. 

GENERAL  PRINCIPLES  OF  BACTERIOLOGY. 


CHAPTER  I. 
STRUCTURE  AND  FORM. 

BEFORE  one  can  gain  any  intelligent  conception  of 
the  manner  in  which  bacteria  affect  dairying,  it  is  first 
necessary  to  know  something  of  the  life  history  of  these 
organisms  in  general,  how  they  live,  move,  and  react 
toward  their  environment. 

1.  What  are  bacteria?  Toadstools,  smuts,  rusts, 
and  mildews  are  known  to  even  the  casual  observer,  be- 
cause they  are  of  such  evident  size.  Their  plant-like 
nature  can  be  readily  understood  from  their  general  struc- 
ture and  habits  of  life.  The  bacteria,  however,  are  so 
small  that,  under  ordinary  conditions,  they  only  become 
evident  to  our  unaided  senses  by  the  by-products  of  their 
activity. 

When  Leeuwenhoek  (pronounced  Lave-en-hake)  in 
1675  first  discovered  these  tiny,  rapidly  moving  organ- 
isms, he  thought  they  were  animals.  Indeed,  under  a 
microscope,  many  of  them  bear  a  close  resemblance  to 
those  minute  worms  found  in  vinegar  that  are  known  as 
"vinegar  eels."  The  idea  that  they  belonged  to  the  ani- 
mal kingdom  continued  to  hold  ground  until  after  the 
middle  of  the  present  century ;  but  with  the  improvement 
in  microscopes,  a  more  thorough  study  of  these  tiny 

structures  was  made  possible,  and  their  vegetable  nature 
i— B.     ' 


2  Dairy  Bacteriology. 

demonstrated.  The  bacteria  as  a  class  are  separated  from 
the  fungi  (smuts,  molds,  etc.) ,  mainly  by  their  method  of 
growth;  from  the  lower  green  algae  by  the  absence  of 
chlorophyll,  the  green  coloring  matter  of  vegetable 
organisms . 

2.  Structure  of  bacteria.     As  far  as  structure  is  con- 
cerned the  bacteria  stand  on  the  lowest  plane  of  vegetable 
life.     They  are  single- celled  plants.     In  its  inner  struc- 
ture, the  cell  does  not  differ  essentially  from  that  of  many 
other  types  of  plant  life.     It  is  composed  of  a  proto- 
plasmic body  which  is  surrounded  by  a  thin  membrane 
that  separates  it  from  neighboring  cells  that  are  alike  in 
form  and  size. 

3.  Form  and  size.     Where  a  plant  is  composed  of  a 
single  cell  but  little  difference  in  form  is  to  be  expected. 
While  there  are  intermediate  stages  that  grade  insensibly 
into  one  another,  the  bacteria  may  be  grouped  into  three 
main  types    (so  far  as  form  is  concerned).     These  are 


FIG.  1.— Different  forms  of  bacteria,  a,  b,  c,  represent  different  types  as  to 
form:  a,  coccus,  b,  bacillus,  c,  spirillum;  d,  diplococcus  or  twin  coccus;  e,  staphy- 
lococcus  or  cluster  coccus;  /  and  g,  different  forms  of  bacilli,  £•  shows  internal 
endospores  within  cell;  h  and  *',  bacilli  with  motile  organs  (cilia). 

spherical,  elongated,  and  spiral,  and  to  these  different 
types  are  given  the  names,  respectively,  coccus,  bacillus, 
and  spirillum  (plural,  cocci,  bacilli,  spirilla),  (fig.  1). 


Structure  and  Form.  3 

A  ball,  a  short  rod,  and  a  corkscrew  serve  as  convenient 
models  to  illustrate  these  different  forms. 

In  size,  the  bacteria  as  a  class  are  the  smallest  organ- 
isms that  are  known  to  exist.  Relatively  there  is  consid- 
erable difference  in  size  between  the  different  species,  yet 
in  absolute  amount  this  is  so  slight  as  to  require  the 
highest  powers  of  the  microscope  to  detect  it.  As  an 
average  diameter,  one-thirty-thousandth  of  an  inch  may 
be  taken.  If  a  hundred  individual  germs  could  be  placed 
side  by  side,  their  total  thickness  would  not  equal  that  of 
a  single  sheet  of  paper  upon  which  this  page  is  printed. 

4.  Manner  of  growth.     As  the  cell  increases  in  size 
as  a  result  of  growth,  it  usually  elongates  in  one  direc- 
tion, and  finally  a  new  cell- wall  is  formed,  dividing  the 
-so- called  mother- cell  into  two  equal- sized  daughter- cells. 
This  process  of  cell  division,  which  is  called,  fission,  is 
continued  almost  indefinitely  until  growth  ceases  as  a  nat- 
ural sequence. 

5.  Cell  arrangement.     If  this  process  of  fission  goes 
on  in  the  same  plane  it  results  in  the  formation  of  a  cell- 
row.     A  species  forming  such  a  chain  of   cells  of   the 
coccus  type  is  called  strepto  -  coccus  (chain- coccus).     If 
the  second  division  plane  is  formed  at  right  angles  to  the 
first,  a  cell -surface  is  formed.     If  growth  takes  place  in 
three  dimensions  of  space,  a  cell -mass  is  produced  as  in 
the  sarcina  group.     In  some  cases  these  cell  aggregates 
cohere  so  tenaciously  that  this  character  is  of  value  in 
distinguishing  different  species. 

6.  Reproduction.     The  process  of  cell  division  known 
as  fission  enables  the  bacterial  form  to  reproduce  itself 
rapidly.  '  Some  species  possess  another  method  of  vege- 
tative  reproduction,    viz.,    spore-formation   (fig.    1,  g) . 
The  spores  are  usually  formed  within  a  mother-cell ;  hence, 


4  Dairy  Bacteriology. 

they  are  called  endospores.  The  protoplasm  of  the  endo- 
spore  contracts  into  a  small  ball,  and  finally  acquires  a 
thickened  outer  part  that  enables  it  to  resist  unfavorable 
surroundings;  therefore,  they  are  adapted  to  the  perpetua- 
tion of  the  species  under  severe  conditions,  and  are  analo- 
gous in  this  respect  to  the  seeds  of  higher  plants.  Many 
of  the  bacilli  form  spores,  the  cocci  do  not.  Spores  do  not 
germinate  and  grow  as  readily  as  the  ordinary  cells. 

7.  Movement.     Many  bacteria  are  unable   to   move 
about  from  place  to  place.     They  have,  however,  a  vibra- 
tory movement  known  as  the  Brownian  motion,  that  is 
purely  physical.     Many  other  forms  possess  an  ability  to 
move  about.     This  they  do  by  means  of  fine  thread-like 
processes  on  the  edge  of  the  cell  known  as  cilia  (sing. 
cilium) .     Coccus  forms  in  general  are  non-motile,  while 
bacilli  may  or  may  not  possess  these  locomotor  organs. 

8.  Classification.     In  arranging  bacteria   according 
to  their  various  relationships,  difficulties  are  experienced 
that  do  not  obtain  with  the  higher   forms  of   plants. 
There  exists  so  little  difference  between  the  various  spe- 
cies as  regards  form  and  size  that  it  becomes  necessary 
to  employ  other  characters  of  a  physiological  nature,  as 
for  instance  the  way  in  which  the  germ  develops  in  cul- 
ture media,  the  by-products  that  are  formed  as  a  result 
of  growth  and  other  characters  of  a  similar  nature. 


CHAPTER  II. 
PHYSIOLOGY. 

9.  Conditions  essential  for  growth.    The  growth 
of  bacteria,  like  all  other  living  organisms,  bears  a  direct 
relation  to  their  external  surroundings.     Certain  condi- 
tions  are  absolutely  essential  before  life  can  develop. 
Other  conditions  though  often  advantageous  are  not  of 
such  vital  importance. 

10.  1.  Food-supply.     Concerning   the   character   of 
the  food- supply  necessary  for  different  species  there  is  a 
very  great  difference.     Many  of   the  disease- producing 
forms  are  very  particular  in  the  selection  of  their  food. 
With  those  forms  that  are  usually  found  in  milk,  such 
delicacy  of  choice  is  not  common. 

A  food  substance  to  be  available  for  bacteria  must  be 
in  solution,  as  it  must  pass  through  the  cell-wall  by 
absorption.  Bacteria  can  live  upon  solid  substances,  but 
they  form  chemical  substances  that  enable  them  to  render 
soluble  the  necessary  food  material. 

The  essential  food  elements  that  must  be  present  are 
nitrogen,  carbon,  and  oxygen,  together  with  minute  quan- 
tities of  mineral  elements.  The  nitrogen  and  carbon  are 
more  available  when  in  the  form  of  organic  compounds 
than  as  simple  inorganic  salts.  Albuminous  or  proteid 
substances  are  best  adapted  for  the  nitrogen  supply,  while 
sugars  are  available  for  the  carbonaceous  part  of  the 
food.  The  nitrogenous  element  is,  however,  the  most 
indispensable. 

Concentration  of  medium.  If  the  fluid  is  too  dense, 
bacteria  cannot  grow.  Condensed  milk  or  syrup  is  too 

[51 


6  Dairy  Bacteriology. 

concentrated  to  permit  of  bacterial  growth,  but  if  diluted 
considerably,  rapidly  undergoes  fermentation.  The  keep- 
ing quality  of  these  products  is  dependent  upon  this  prop- 
erty. 

Chemical  reaction  of  medium.  As  a  rule,  bacteria  prefer 
a  slightly  alkaline  to  an  acid  medium,  but  there  are  so 
many  exceptions  to  this  rule,  that  the  force  of  it  as  a  gen- 
eral statement  is  not  very  strong.  Those  organisms  that 
are  normal  inhabitants  of  milk  are,  as  a  rule,  less  sus- 
ceptible to  slight  variations  in  the  reaction  of  the  food 
medium  than  many  others. 

11.  2.  Temperature.  A  certain  degree  of  heat  is  ab- 
solutely necessary  before  the  spores  of  bacteria  can  ger- 
minate, just  as  seed  grain  will  not  sprout  when  the  ground 
is  too  cold.  As  the  temperature  of  a  fluid  increases,  the 
rapidity  with  which  the  bacteria  multiply  also  increases 
for  a  time.  Beyond  a  certain  point,  however,  a  heat  rigor 
sets  in  that  destroys  the  activity  of  the  protoplasm.  There 
is  therefore,  an  optimum  or  best  temperature  for  growth, 
and  minimum  and  maximum  points  as  well, below  and  above 
which,  development  is  impossible.  These  three  cardinal 
growth  points  vary  considerably  with  different  germs. 

The  temperature  limits  of  growth,  i.  e.,  the  range  be- 
tween the  maximum  and  minimum  points  of  development, 
are  much  wider  with  bacteria  than  with  almost  any  other 
forms  of  living  matter.  For  this  reason,  bacteria  are 
more  widely  distributed  than  any  other  class  of  living 
organisms. 

Some  forms  thrive  at  32°  F.,  while  others  are  able  to 
grow  at  temperatures  approximating  140°  F.  With  the 
great  majority  of  bacteria,  especially  those  growing  in 
milk,  this  range  is  not  so  great.  Most  milk  bacteria  fail 
to  develop  at  a  point  below  40°-50°  F . ,  while  the  maximum 
growth  point  does  not  exceed  105°-110°  F.,  the  optimum 


Physiology.  1 

ranging  from  80°-100°  F.  Parasitic  forms  living  in  the 
animal  or  human  body  have  a  high  optimum,  usually  ap- 
proximating the  blood  temperature  (98°-100°  F.). 

12.  3.  Gaseous  environment.    To  most  forms  of  life, 
atmospheric  air  is  a   necessity,  in  order  to    supply  the 
oxygen  used  in  growth.     With  the  bacteria,  the -great 
majority  require  the  free  access  of  air  the  same  as  other 
living  organisms,  and  if  denied  this,  fail  to  grow.     Such 
bacteria  are  called  aerobic.     Toward  some,  however,  oxy- 
gen acts  as  a  direct  poison.     Only  when  they  are  sur- 
rounded with   an  atmosphere  other  than   air,   such   as 
hydrogen  or  nitrogen  gas,  can  they  grow.     These  forms 
are  called  anaerobic.     All  bacteria  are  not  divided  sharply 
into  these  two  classes.     While  some  of  them  grow  strictly 
under  one  condition  or  the  other,  hence  are  called  obligate 
aerobes  or  anaerobes,  other  forms  are  seemingly  indiffer- 
ent to  their  gaseous    surroundings.     To  this  class,  the 
name  of  facultative  or  optional  aerobe  or  anaerobe  is  given, 
depending  upon  the  relation  of  the  germ  to  the  oxygen 
supply  (fig.  1,  d,  e) . 

Most  milk  bacteria  belong  to  the  aerobic  class,  but 
there  are  quite  a  number  that  are  able  to  grow  without 
free  oxygen. 

13.  4.  Moisture.     On  a  dry  medium,  bacteria  can  not 
grow,  neither  can  the  spores  germinate.  A  certain  amount 
of  moisture  is,  therefore,  necessary  before  bacterial  growth 
can  take  place.     Organic  substances  such  as  vegetable  or 
animal  tissue  contain  sufficient  water  to  permit  growth. 

14.  Rate  of  growth.     The  rate  of  growth  of  actively 
vegetating  bacteria  is  in  many  instances  perfectly  astound- 
ing.    With  many  species  under  favorable  conditions,  a 
single  cell  will  divide  in  twenty  minutes,  and  each  of  the 
daughter- cells  will   repeat  this   operation    in   the  same 


8  Dairy  Bacteriology. 

time.  This  rapid  rate  cannot  be  maintained  indefinitely, 
for  the  bacteria  soon  limit  their  own  development  by  the 
production  of  by-products  that  are  unfavorable  to  their 
own  growth.  The  sour  milk  bacillus  thrives  readily  in 
milk  until  the  lactic  acid  that  is  formed  by  it  exceeds  a 
certain  amount,  then  growth  ceases.  If  this  acid  is  neu- 
tralized by  the  addition  of  chalk,  the  lactic  bacteria  will 
start  again  to  grow,  and  produce  acid  to  the  point  of 
excess.  There  is  a  marked  difference  with  different 
forms  in  their  rate  of  growth,  and  the  conditions  under 
which  development  best  occurs. 

15.  Effect  of  external  conditions.     Bacteria,  even 
in  a  vegetating  stage,  possess  a  much  greater  resistance 
toward  external  forces  than  other  forms  of  animal  or  veg- 
etable life.     When  they  are  in  a  spore  stage,  this  resist- 
ance is  still  further  increased.     A  thorough  knowledge 
of  the  effect  of  these  external  forces  is  essential,  for  it  is 
by  their  action  that  wre  are  often  able  to  destroy  undesir- 
able forms  of  germ  life. 

16.  Physical  forces.     1.  Heat.     The  bacteria  possess 
a  high  power  of  resistance  toward  heat ;  but  this  relation 
varies,  depending  upon  the  condition  in  which  it  is  ap- 
plied, whether  it  is  in  a  dry  or  moist  state.     At  some 
temperatures  moist  heat,  on  account  of  its  greater  pene- 
trative power,  is  much  more  effective  than  dry  in  destroy- 
ing germ  life.     The  temperature  at  which  any  form  is 
killed  is  called  its  thermal  death  point.     For  the  majority 
of  germs  in  a  spore-free,  developing  condition,  a  temper- 
ature from   130°-140°   F.  for  ten  minutes  in  a  liquid 
medium  is  fatal.     A  shorter  exposure  necessitates  a  some- 
what higher  temperature. 

When  in  the  more  resistant  spore- stage,  many  of  them 
are  able  to  withstand  moist  heat  in  the  form  of  steam 
(212°  F.)  from  one  to  three  hours.  To  destroy  spores 


Physiology.  9 

by  dry  heat,  a  temperature  varying  from  260°-300°  F.  is 
necessary  for  an  hour  or  more.  Germ  life  may  be  more 
rapidly  destroyed  in  super-heated  steam  under  pressure, 
a  temperature  of  230°-240°  F.  for  fifteen  to  twenty  min- 
utes being  sufficient  to  kill  most  species.  Many  of  the 
milk  bacteria,  like  the  sour  milk  germ,  are  very  easily 
destroyed  by  heat,  as  they  do  not  form  spores.  Other 
forms  like  the  hay  and  potato  bacilli  are  difficult  to 
eradicate  on  account  of  the  great  resistance  that  their 
spores  offer  toward  heat.  High  temperatures  are  by  far 
the  most  efficient  means  that  can  be  used  to  render  any 
substance  germ-free  and  are  most  often  employed  for 
this  purpose. 

2.  Cold.     While  the   maximum  temperature  that  the 
different  spore-bearing  species  can  stand  has  been  accu- 
rately determined  in  many  cases,  the  minimum  tempera- 
ture has  in  most  instances  never  been  reached.     But  lit- 
tle reliance  is  to  be  placed  upon  this  method  in  attempting 
to  free  any  substance   completely  from   bacterial   life, 
although  freezing  is  able  to  destroy  a  large  percentage. 
Even  an  artificial  degree  of  cold  as  low  as  —  220°  F.  for 
a  day  has  been  found  to  be  insufficient  to  destroy  some 
forms,    especially  when  in   the  spore    stage.     Bacterial 
growth  is  held  in  abeyance  when  a  liquid  is  congealed, 
but  development  occurs  very  soon  after  it  is  melted. 

3.  Desiccation.     Different  bacteria  behave  very  differ- 
ently when  subjected  to  drying.     The  cholera  germ  dies  in 
three  hours  if  it  is  dried,  while  anthrax  retains  its  viru- 
lence unimpaired  for  decades.     Tuberculous  sputum  with- 
stands drying  and  is  often  found  to  be  infectious  after 
the  lapse  of  eight  or  nine  months.     Those  species  that 
form  spores  naturally  resist  desiccation  much  better  than 
those  that  do  not  form  these  latent  structures. 


10  Dairy  Bacteriology. 

4 .  LigM .  The  influence  of  light  on  bacterial  growth  has 
not  been  generally  appreciated  until  within  a  few  years. 
Exposed  to  the  rays  of  direct  sunlight,  many  forms  are 
killed  in  a  few  hours.  Even  diffused  daylight  often  ex- 
erts a  powerful  inhibiting  effect,  if  the  exposure  covers 
any  considerable  length  of  time.  Bacterial  spores  are  not 
as  readily  destroyed  as  the  vegetative  forms. 

17.  Chemical  substances.     Different  chemical  sub- 
stances exert  a  powerful  toxic  action  on  bacterial  life 
with  which  they  come  in  contact.     Those  that  destroy  or 
kill  bacterial  forms  are  known  as  disinfectants;   those 
that  merely  inhibit,  or  retard  growth  are  known  as  anti- 
septics.     All  substances  possessing  disinfecting  power 
must  of  necessity  be  antiseptic  in  their  action,  but  not 
all  antiseptics  are  disinfectants  even  when  used  in  strong 
doses.     Most  of  the  disinfectants,  under  ordinary  cir- 
cumstances, have  no  use  in  creamery  practice  except  as 
preservatives  for  samples.     Of  these  corrosive  sublimate 
and  potassium  bichromate  are  most  frequently  used.     In 
some  countries  antiseptics,  principally  of  boracic  acid  or 
formalin,  are  employed  to  "keep"  milk,  but  in  general, 
their  use  is  prohibited  by  law. 

18.  The  role  of  bacteria  in  nature.     The  great  ma- 
jority of  bacteria  like  animal  life,  belong  to  a  class  whose 
function  it  is  to  disintegrate  organic  matter  and  resolve 
it  into  its  constituent  elements.  The  value  of  this  break- 
ing down  process  is  evident  at  a  glance  when  we  consider 
what  would  be  the  result,  if  all  decay,  putrefaction,  and 
decomposition  were  at  once  arrested.     Not  only  would 
the  supply  of  carbonic  acid  gas  be  very  soon  absorbed, 
stopping  the  development  of  chlorophyll-bearing  plants, 
but  all  nature  would  be  clogged  and  soon  buried  under 
its  own  debris.     Dead  and  dying  vegetable  and  animal 


Physiology.  11 

matter,  unable  to  rot  and  decay  would  soon  bury  all  un- 
der the  constantly  accumulating  mass.  Owing  to  the 
energies  of  the  lower  types  of  plant  and  animal  life,  all 
of  this  dead  effete  matter  is  slowly  consumed.  The  split- 
ting of  these  materials  into  the  simpler  elements  is  largely 
due  to  the  activities  of  the  bacteria.  Almost  without  ex- 
ception, they  prey  upon  organized  matter,  absorbing  suf- 
ficient energy  for  their  maintenance,  and  at  the  same  time, 
giving  off  the  unused  elements  to  form  new  combinations 
that  are  again  absorbed  in  different  forms. 

19.  Specialization  of  bacteria.  With  the  higher  plant 
and  animal  forms,  a  certain  specialization  is  in  progress. 
The  plant  or  the  animal  adapts  itself  to  its  environment, 
and  in  doing  so,  each  group  as  a  whole  acquires  certain 
characteristics  that  mark  it  in  a  greater  or  lesser  degree. 
Usually,  specialization  accompanies  a  complexity  of  struc- 
ture, so  that  the  higher  developed  plants  and  animals  show 
this  characteristic  more  fully  than  those  that  belong  to 
the  more  primitive  types. 

In  the  bacteria  is  seen  a  curious  anomaly  to  this  rule. 
Simple  in  structure  as  they  appear  to  be,  they  surpass  in 
functional  variability  many  of  the  higher  forms  of  plant 
and  animal  life.  Ubiquitous  in  their  distribution  through- 
out the  realm  of  nature,  they  have  gradually  adapted 
themselves  to  the  different  habitats  in  which  they  have 
found  themselves,  and  as  a  result,  more  or  less  well  marked 
groups  are  to  be  found.  Thus,  there  is  a  normal  bacte- 
rial flora  of  the  mouth,  another  of  the  skin,  and  still 
another  of  the  intestines.  Again,  there  is  a  class  known 
collectively  as  water  bacteria,  still  another  that  might 
with  propriety  be  classified  as  milk  bacteria.  Those  forms 
that  have  become  habituated  to  certain  conditions,  may 
be  said  to  be  indigenous  or  native  to  that  habitat,  while 
the  presence  of  uncommon  forms  from  some  other  source 


12  Dairy  Bacteriology. 

would  be  considered  as  adventive  or  introduced.  Thus, 
the  tubercle  bacillus  is  adventive  in  milk,  even  though  it 
maybe  derived  from  the  udder  direct,  while  the  sour  milk 
bacillus  is  a  natural  and  therefore  indigenous  organism. 
20.  Distribution  of  bacteria.  If  one  classifies  bac- 
teria according  to  the  habitat  in  which  they  are  found, 
they  may  be  divided  into  various  groups  such  as  soil,  air 
or  water  forms. 

1 .  Soil  bacteria .     The  superficial  layers  of  the  soil  teem 
with  myriads  of  forms  of  bacteria.     The  amount  of  food 
present  and  other  favorable   growth  conditions  enable 
many  forms  to  thrive  luxuriantly.     At  a  depth  of  a  few 
feet  most  of  them  are  filtered  out,  and  the  conditions  are 
likewise  unsuitable  for  their  growth .  so  in  deep  layers  the 
soil  is  practically  sterile. 

2.  Air  bacteria.     The  bacteria  found  in  the  air  are  orig- 
inally from  the  soil  beneath.     In  the  atmosphere  they  are 
unable  to  develop,  but  exist  in  a  latent  condition.     Their 
prevalence  in  the  air  is  measured  by  the  condition  of  the 
soil  below  and  the  movement  of  dust  particles.     For  this 
reason  they  are  more  numerous  in  summer  than  in  win- 
ter;  the  atmosphere  of  cities  contains  a  larger  number  of 
them  than  country  air.     They  are  very  prevalent  in  illy- 
ventilated  houses  and  out-buildings,  particularly  barns 
and  stables,  where  dust  from  hay  and  dried  manure  par- 
ticles fill  the  air. 

3.  Water  bacteria.     Water  when  exposed  to  the  air 
invariably  contains  a  sufficient  amount  of  organic  matter 
to  serve  as  bacterial  food.     Some  of  its  germ  life  is  de- 
rived from  dust,  or  the  washings  of  the  land,  but  many 
species  exist  in  this  element  which  are  not  to  be  found  in 
soil.    Stagnant  pools  rich  in  organic  matter  always  teem 
with  bacteria,  and  even  running  water  often  has  large 
numbers.     The  ground  water  layer  is  normally  free,  as 


Physiology.  13 

the  bacteria  are  filtered  out  by  passing  through  the  inter- 
vening soil  layers.  As  a  consequence  spring  water  as  it 
issues  from  the  soil  is  relatively  poor  in  bacteria,  but 
quickly  becomes  contaminated  after  it  reaches  the  sur- 
face. Some  of  the  highly  infectious  diseases,  such  as 
cholera  and  typhoid  fever  are  often  transmitted  by  means 
of  a  contaminated  water-supply. 

21.  Saprophytic  bacteria.    If  bacteria  are  consid- 
ered from  the  manner  in  which  they  live,  they  may  be 
divided  into  two  very  unequal  divisions,  known  as  sapro- 
phytes and  parasites.     Those  belonging  to  the  first  class 
subsist  on  dead  organic  matter,  while  the  parasitic  forms 
are  able  to  thrive  in  living  tissues  of  either  vegetable  or 
animal  nature.      The  great  majority  of  different  forms 
belong  to  the  first  class,  and  it  is  undoubtedly  true  that 
this  condition  is  more  fundamental  than  the  parasitic 
method  of  life.     The  saprophytes  find  their  food- supply 
in  the  vegetable  and  animal  matter  that  has  already  ceased 
to  live.     They  are  the  organisms  concerned  in  the  tear- 
ing down  processes  of  nature  and  their  beneficent  func- 
tion in  the  world  about  them  is  in  their  scavenger  char- 
acter as  they  make  way  with  the  offal  and  debris  of 
organic  life. 

22.  Parasitic  bacteria.     There  is  no  sharp  line  sep- 
arating the  parasitic  bacteria  from  those  that  live  on 
lifeless  matter.     In  all  probability  these  forms  that  are 
now  able  to  thrive  in  living  tissue  came  originally  from 
ancestors  of  a  saprophytic  type.     Some,  as  the  tubercle 
bacillus,  have  gradually  adapted  themselves  to  the  more 
restricted  parasitic  method  of  growth,  just  as  many  para- 
sitic plants  have  been  produced,  and  as  a  result  of  this 
specialization,  they  have  often  lost  the  power  to  thrive 
under  conditions  generally  favorable  to  saprophytic  forms. 

In  parasitism  a  marked  variation  in  degree  is  to  be 


14  Dairy  Bacteriology. 

noted.  Some  species,  like  leprosy,  grow  with  only  the 
greatest  difficulty  outside  of  their  proper  host.  Such  as 
this  may  be  said  to  be  an  obligatory  parasite.  Then 
again,  other  forms  like  the  colon  or  feces  bacillus  are 
normally  saprophytes,  but  under  certain  conditions,  as- 
sume a  semi-parasitic  mode  of  life,  becoming  therefore, 
facultative  parasites,  i.  e.,  they  possess  at  times  the  fac- 
ulty of  developing  under  parasitic  conditions. 

23.  Fermentation.  Most  of  the  saprophytic  bacteria 
are  concerned  in  the  breaking  down  of  organic  matter, 
and  consequently,  they  are  often  associated  with  the 
many  different  phases  that  this  process  assumes  as  seen 
in  putrefaction,  fermentation,  decomposition  or  decay. 
These  changes  are  of  a  complex  character  and  vary  much 
from  one  instance  to  another. 

The  changes  embraced  under  the  special  term,  fermen- 
tation, are  distinguished  by  such  a  prominent  character- 
istic that  they  may  well  be  considered  separately.  In 
fermentation,  complex  substances  are  transformed  by 
regular  steps  into  simpler  compounds. 

In  this  class  are  to  be  included  a  large  number  of  fer- 
mentative changes  that  occur  in  milk,  such  as  the  produc- 
tion of  lactic  acid  in  the  souring  of  milk,  the  formation  of 
butyric  acid,  and  others  that  will  be  mentioned  later.  In 
fermentation,  there  are  often  large  quantities  of  the  fer- 
mentable substances  changed,  and,  while  a  certain  amount 
of  the  energy  released  in  the  breaking  down  of  this  ma- 
terial may  be  utilized  by  the  bacterial  cells,  yet  the  dis- 
ruptive changes  set  in  motion  by  the  living  germs  are 
out  of  all  proportion  to  the  results  that  are  seen  in  the 
end. 

Fermentation,  although  one  of  the  best  known  pro- 
cesses that  occurs  in  nature  is  even  yet  a  partial  mystery. 
Our  knowledge  of  the  changes  that  fermentable  solutions 


Physiology.  15 

undergo  during  this  process  has  been  vastly  augmented 
during  the  latter  half  of  this  century,  but  even  yet,  the 
problem  has  not  been  completely  solved.  The  results  of 
fermentation  have  been  made  use  of  from  time  immemo- 
rial, but  no  adequate  conception  of  the  changes  involved 
was  ever  recognized  until  this  century.  The  earlier  the- 
ories of  this  action  were  mainly  chemical,  and  it  was  not 
until  the  important  studies  of  Pasteur  were  made  that 
the  relation  of  these  changes  to  the  action  of  living  or- 
ganisms was  fully  proven.  He  showed  that  fermentation 
was  closely  connected  with  the  growth  and  multiplication 
of  minute  forms  of  organic  life,  and  that  the  process  was 
usually  inaugurated  by  vital  forces  rather  than  by  purely^ 
chemical  activity. 

Previous  to  his  work  the  action  of  rennet  upon  milk 
had  been  known,  and  the  difference  between  this  fermen- 
tation and  other  types  had  been  recognized  by  Schroeder 
and  Dusch.  They  showed  that  the  rennet  ferment  was 
unaffected  by  alcohol  and  other  chemicals,  while  other 
fermentations  due  to  organisms  were  checked  in  these 
fluids.  Pasteur  emphasized  the  distinction  between  these 
two  sets  of  fermentative  action  and  soon  was  able  to  clas- 
sify these  changes  into  two  distinct  types. 

1.  Organized  or  living  ferments  —  those  in  which  the 
fermentative  change  takes  place  as  a  result  of  the  activity 
of  a  living  organism. 

2.  Unorganized  or  chemical  ferments  —  those  in  which 
the  change  is  caused  by  a  chemical  substance,  devoid  of 
vitality,  that  is  itself  unchanged  in  the  fermenting  pro- 
cess.    These  unorganized,  non- vital  ferments  are  known 
as  enzymes.     Among  the  better  known  of  these  are  ren- 
net, that  has  the  power  of  coagulating  milk;   diastase, 
the  enzyme  that  converts  starch  into  sugar;   pepsin  and 
trypsin,  the  digestive  ferments  of  the  animal  body.     In 


16  Dairy  Bacteriology. 

their  relation  toward  external  influences  such  as  heat, 
etc.,  many  of  these  are  affected  in  a  manner  similar  to 
the  vital  ferments. 

Among  the  organized  class  of  ferments  are  those  that 
directly  affect  the  character  of  sugar- containing  fluids, 
such  as  the  yeasts  and  most  of  the  bacteria.  A  great 
many  of  these  organized  ferments  accomplish  their  effect 
indirectly  through  the  agency  of  the  enzymes  that  they 
themselves  excrete.  For  instance,  many  of  the  abnormal 
fermentations  in  milk  are  caused  by  bacteria  that  are  able 
to  form  various  enzymes . 


CHAPTER  III. 
METHODS  OF  STUDYING  BACTERIA. 

24.  Necessity  of  bacterial  masses  for  study.  Bac- 
teria are  so  infinitesimally  small  that  it  is  impossible  to 
study  individual  germs  separately  without  the  aid  of  first- 
class  microscopes.     For  this  reason,  but  little  advance 
was  made  in  the  knowledge  of  these  lower  forms  of  plant 
life,  until  the  introduction  of  culture  methods,  whereby 
a  single  organism  could  be  cultivated  and  the  progeny  of 
this  cell  increased  to  such  an  extent  in  a  short  course  of 
time,  that  they  would  be  visible  to  the  unaided  eye. 

25.  Culture   methods.     The   system  of   cultivating 
bacteria,  known  as  the  pure  culture  method,  is  based 
upon  the  supposition  that  the  food  medium  in  which  the 
organism  is  grown  is  first  freed  completely  from  all  pre- 
existing forms  of  life,   or  in  other  words,   is  perfectly 
sterile.     The  pure  culture  processes  of  the  bacteriologist 
may  be  said  to  be  in  a  sense,  refined  methods  of  seeding, 
such  as  the  agriculturist  employs .  Just  as  the  seed  grain  will 
in  due  season  bring  forth  a  harvest  after  its  kind,  so  any 
kind  of  bacteria  planted  in  a  favorable  food  medium  will 
produce  a  crop  of  its  own.     If  the  farmer's  seed  is  foul, 
it  shows  in%is  crop,  and  the  same  is  true  with  bacterial 
farming. 

Bacteria,  however,  are  so  universally  distributed  that 
it  becomes  an  impossibility  to  grow  any  special  kind, 
unless  the  soil  is  first  freed  from  all  existing  forms  of 
germ  life.  To  accomplish  this,  it  is  necessary  to  subject 
the  nutrient  medium  used  for  a  culture  to  some  method 
of  sterilization,  such  as  by  heat  or  filtration,  whereby  all 

2-B.  I 


18  Dairy  Bacteriology. 

forms  of  organic  life  are  thoroughly  eliminated.  Germ- 
free  culture  material  is  kept  in  sterilized  glass  tubes  and 
flasks,  and  is  protected  from  outside  infection  by  plugs 
of  sterile  cotton.  Material  thus  prepared,  if  protected 
from  evaporation,  will  keep  indefinitely,  as  the  cotton 
acts  as  an  effectual  filter  against  the  passage  of  any  par- 
ticles of  matter. 

26.  Culture  media.  For  culture  media,  many  differ- 
ent substances  are  employed.  In  fact,  bacteria  will  grow 
on  almost  any  organic  substance  whether  it  is  solid  or 
fluid,  provided  the  essential  conditions  of  growth  are  fur- 
nished. The  food  substances  that  are  used  for  culture 
purposes  are  divided  into  two  classes;  solids  and  liquids. 

Solid  media  may  be  either  permanently  solid  like  pota- 
toes or  they  may  retain  their  solid  properties  only  at  cer- 
tain temperatures  like  gelatin  or  agar.  These  last  are  of 
utmost  importance  in  bacteriological  research,  for  their 
use,  which  was  introduced  b(y  Koch,  permits  the  separa- 
tion of  the  different  forms  that  may  happen  to  be  in  any 
mixture.  Gelatin  is  used  advantageously  because  the 
majority  of  bacteria  present  wider  differences  in  their  ap- 
pearance upon  this  medium  than  upon  any  other.  It  re- 
mains solid  at  ordinary  temperatures,  becoming  liquid  in 
the  neighborhood  of  70°  F.  Agar,  a  gelatinous  product 
derived  from  a  Japanese  sea- weed,  has  a  much  higher 
melting  point,  and  can  be  successfully  used,  especially 
with  those  organisms  whose  optimum  growth  point  is 
above  the  melting  point  of  gelatin. 

Besides  these  solid  media,  different  liquid  substances 
are  extensively  used,  such  as  beef  broth,  milk,  and  in- 
fusions of  various  vegetable  and  animal  tissues.  Skim- 
milk  is  of  especial  value  in  studying  the  milk  bacteria 
and  may  be  used  in  its  natural  condition,  or  a  few  drops 
of  litmus  solution  may  be  added  in  order  to  detect  any 
change  in  its  chemical  reaction  due  to  the  bacteria. 


Methods  of  Studying  Bacteria. 


19 


27.  Methods  of  isolation.  Suppose  for  instance  one 
wishes  to  isolate  the  different  varieties  of  bacteria  found 
in  milk.  The  method  of  procedure  is  as  follows:  Sterile 
gelatin  in  glass  tubes  is  melted  and  cooled  down  so  as  to 
be  barely  warm.  To  this  gelatin  which  is  germ-free  a 
drop  of  milk  is  added.  The  gelatin  is  then  gently  shaken 
so  as  to  thoroughly  distribute  the  milk  particles,  and 
poured  out  into  a  sterile  flat  glass  dish  and  quickly  cov- 


b--- 


FIG.  2.  A  gelatin  plate  culture  showing  appearance  of  different  organisms 
from  a  sample  of  milk.  Each  mass  represents  a  bacterial  growth  (colony)  de- 
rived froia  a  single  cell.  Different  forms  react  differently  toward  the  gelatin, 
some  liquefying  the  same,  others  growing  in  a  restricted  mass,  a,  represents  a 
colony  of  the  ordinary  bread  mold;  b,  a  liquefying  organism;  c,  and  d,  solid 
forms. 


ered.  This  is  allowed  to  stand  on  a  cool  surface  until 
the  gelatin  hardens.  After  the  culture  plate  has  been 
left  for  twenty- four  to  thirty- six  hours  at  the  proper 
temperature,  tiny  spots  will  begin  to  appear  on  the  sur- 
face, or  in  the  depth  of  the  culture  medium.  These 


20 


Dairy  Bacteriology. 


patches  are  called  colonies  and  are  composed  of  infinite 
numbers  of  individual  germs,  the  result  of  the  continued 
growth  of  the  single  organism  that  was  in  the  drop  of  milk 
which  was  firmly  held  in  place  when  the  gelatin  solidi- 
fied. The  number  of  these  colonies  represent  in  gen- 
eral the  number  of  germs  that  were  present  in  the  milk 
drop.  If  the  plate  is  not  too  thickly  sown  with  these 
germs,  the  colonies  will  continue  to  grow  and  increase  in 


FIG.  3.  Profile  view  of  gelatin  plate  culture.  Shaded  part  represents  the 
gelatin  medium  in  the  covered  glass  dish;  on  the  surface,  different  bacteria 
are  developing;  b,  is  a  liquefying  form  that  dissolves  the  gelatin  while  c  and  d 
grow  on  surface  only  and  do  not  render  gelatin  soluble. 

size,  and  as  they  do,  minute  differences  will  begin  to  ap- 
pear. These  differences  may  be  in  the  color,  the  con- 
tour and  the  texture  of  the  colony,  or  the  manner  in 
which  it  acts  toward  gelatin.  (See  fig.  3.)  In  order 
to  make  sure  that  the  seeding  is  not  too  copious  so  as  to 
interfere  with  continued  study,  an  attenuation  is  usually 
made.  This  consists  in  taking  a  drop  of  the  infected 
gelatin  in  the  first  tube,  and  transferring  it  by  means  of 
a  sterile  needle  into  another  tube  of  sterile  media.  Usu- 
ally this  operation  is  repeated  again  so  that  these  culture 
plates  are  made  with  different  amounts  of  seed  with  the 
expectation  that  in  at  least  one  plate  the  seeding  will  not 
be  so  thick  as  to  prevent  further  study. 

To  further  study  the  peculiarities  of  different  germs, 
the  separate  colonies  are  transferred  to  other  sterile  tubes 


Methods  of  Studying  Bacteria. 


21 


of  culture  material  and  thus  pure  cultures  of  the  various 
germs  are  secured.  These  cultures  then  serve  as  a  basis 
for  continued  study  and  must  be  planted  and  grown  upon 
all  the  different  kinds  of  media  that  are  obtainable.  In 
this  way,  the  slight  variations  in  the  growth  of  different 


1 


FIG.  4.  Pure  cultures  of  different  kinds  of  bacteria  in  gelatin  tubes,  a, 
growth  slight  in  this  medium;  b,  growth  copious  at  and  near  surface.  Fine 
parallel  filaments  growing  out  into  medium  liquefying  at  surface;  c,  a  rapid 
liquefying  form;  d,  a  gas-producing  form  that  grows  equally  well  in  lower  part 
of  tube  as  at  surface  (facultative  anaerobe);  e,  an  obligate  anaerobe,  that  devel- 
opes  only  in  absence  of  air. 

forms  are  detected  and  the  peculiar  characteristics  are 
determined,  so  that  the  student  is  able  to  recognize  this 
form  when  he  meets  it  again. 


22  Dairy  Bacteriology. 

These  culture  methods  are  of  essential  importance  in 
bacteriology,  as  it  is  the  only  way  in  which  it  is  possible 
to  secure  a  quantity  of  germs  of  the  same  kind. 

28.  Use  of  the  microscope  in  bacterial  investiga- 
tion. The  microscope  is  in  constant  demand  throughout 
all  the  different  stages  of  the  isolating  process  in  order 
to  verify  the  purity  of  the  cultures.  For  this  purpose, 
it  is  essential  that  the  instrument  used  shall  be  one  of 
strong  magnifying  powers  (600-800  diameters)  combined 
with  sharp  definition,  so  that  these  tiny  organisms  shall 
stand  out  clear  and  distinct. 

The  microscopical  examination  of  any  germ  is  quite  as 
essential  as  the  culture  characteristics;  in  fact,  the  two 
must  always  go  hand  in  hand.  This  examination  reveals 
not  only  the  form  and  size  of  the  individual  germ,  but 
the  manner  in  which  they  are  united  with  each  other, 
and  any  peculiarities  of  movement  that  they  may  possess. 

In  carrying  out  the  microscopical  part  of  the  work,  not 
only  is  the  organism  examined  in  a  living  condition,  but 
stained  preparations  are  made  by  using  solutions  of  anilin 
dyes  as  staining  agents.  These  are  of  great  service  in 
bringing  out  almost  imperceptible  differences.  The  art 
of  staining  has  been  carried  to  the  highest  degree  of  per- 
fection in  bacteriology,  especially  in  the  detection  of 
germs  that  are  found  in  diseased  tissues  in  the  animal  or 
human  body. 

In  studying  the  peculiarities  of  any  special  organism, 
not  only  is  it  necessary  that  these  cultural  and  micro- 
scopical characters  should  be  closely  observed,  but  special 
experiments 'must  be  carried  out  along  different  lines,  in 
order  to  determine  any  special  properties  that  the  germ 
may  possess.  Thus,  the  ability  of  any  form  to  act  as  a 
fermentative  organism  can  be  tested  by  fermentation  ex- 
periments; the  property  of  causing  disease,  studied  by 


Methods  of  Studying  Bacteria.  23 

the  inoculation  of  pure  cultures  into  animals.  A  great 
many  different  methods  have  been  devised  for  the  pur- 
pose of  studying  special  characteristics  of  different  bacteria, 
but  a  full  description  of  these  would  necessarily  be  so 
lengthy  that  in  a  work  of  this  character  they  must  be 
omitted.1 

i  The  following  general  works  contain  more  or  less  complete  descriptions  of 
the  various  processes  employed  in  studying  bacteria: 

Sternberg,  Manual  of  Bacteriology,  1898;  Frankel,  Bacteriology,  1891;  Wood- 
head,  Bacteria  and  their  products,  1893;  Abbott,  Principles  of  Bacteriology, 
1896;  Pearmain  and  Moor,  Applied  Bacteriology,  1897;  McFarland,  Text-Book 
upon  Pathogenic  Bacteria,  1898. 


PART  II. 

BACTERIA  IN  RELATION  TO  MILK. 

CHAPTER  IV. 
CONTAMINATION  OF  MILK. 

29.  Milk  as  a  food  for  bacteria.  The  fact  that 
milk  so  readily  undergoes  decomposition  changes  shows 
that  it  is  well  suited  for  the  nourishment  of  bacterial 
life.  Its  high  content  in  organic  matter,  and  the  dilu- 
tion of  the  same  in  a  watery  medium  makes  it  an  excel- 
lent food  for  germ  as  well  as  mammalian  life.  Its  dif- 
ferent constituents,  however,  possess  different  nutritive 
values.  Of  most  importance  are  the  nitrogen-containing 
compounds. 

The  albumen  which  is  in  solution  is  readily  available. 
Casein  can  not  be  appropriated  on  account  of  its  insol- 
uble nature,  unless  it  is  first  rendered  soluble,  a  pro- 
cess which  occurs  with  those  bacteria  that  secrete  en- 
zymes that  act  on  proteids. 

Of  the  constituents  of  the  milk  that  belong  to  the  car- 
bon-containing compounds,  only  one  can  be  utilized  by 
bacteria.  The  fat  possesses  but  little  food  value  for  these 
organisms,  because  it  cannot  be  decomposed  by  them. 
The  milk  sugar,  however,  is  an  admirable  food  for  many 
species,  more  especially  those  that  are  known  as  the  lac- 
tic acid  producing  or  natural  milk  bacteria. 

The  bacterial  cell  contains  so  little  mineral  matter  that 
the  requirements  of  the  cell  for  its  growth  are  very  lim- 
ited, yet  the  mineral  elements  of  the  milk  are  needed  for 
the  growth  of  any  protoplasm,  and  are  used  by  the  bac- 
teria in  the  formation  of  new  cell  matter. 

[24] 


Contamination  of  Milk. 


25 


30.  Milk,  germ-free  in  udder.     Under  ordinary  con- 
ditions,  when   examined   in   the   proper   manner,   milk 
always  reveals  bacterial  life.     This  germ  content,  how- 
ever, is  due  to  infection  from  without,  for  in  the  udder  of 
a  healthy  animal,  as  secreted,  the  milk  like  the  other 
secretions  and  tissues  of  the  body  is  normally  sterile. 

31.  Contamination    of   milk.     In   withdrawing  the 
milk  from  the  udder,  it  invariably  comes  in  contact  with 
germ  life.     The  same  is  true  after  it  is  milked.     From 
the  time  of  milking,  until  it  is  consumed  in  one  form  or 
another,  it  is  continually  subject  to  contamination  from 
exterior  sources.     In  the  main,  germ  life  gains  access 
while  the  milk  is  on  the  farm,  but  even  in  the  factory 


FIG.  5. — Microscopic  appearance  of  milk  showing  relative  size  of  fat  globules 
and  bacteria.    Group  of  bacteria  on  left  are  lactic  acid  organisms. 

the  opportunities  for  infection  are  present  in  greater  or 
lesser  degree.  Those  forms  that  gain  an  early  entrance 
generally  predominate  in  the  milk,  as  their  early  intro- 
duction enables  them  to  develop  for  a  longer  period  of 
time. 


26  Dairy  Bacteriology. 

A.     INFECTION  OF  MILK  ON  THE    FARM. 

32.  Sources  of  contamination*     The  bacterial  life 
that  finds  its  way  into  the  milk  while  it  is  yet  on  the  farm 
may  be  traced  to  several  sources,  which  may  be  grouped 
under  the  following  heads:   Unclean  dairy  utensils,  fore 
milk,  coat  of  animal,  and  general  atmospheric  surround- 
ings.    The  relative  importance  of  these  various  factors 
fluctuates  in  each  individual  instance. 

33.  Dairy  utensils.     Of  first  importance,  are  the  ves- 
sels that  are  used  during  milking,  and  also  all  storage  cans 
and  other  dairy  utensils  that  come  in  contact  with  the 
milk  after  it  is  drawn.     By  unclean  utensils,  actually 
visible  dirt  need  not  always  be  considered,  although  its 
presence  in  cracks  and  joints  of  pails  and  cans  is  often 
evident.     Unless  cleansed  with  especial  care,  these  places 
are  apt  to  be  filled  with  foul  and  decomposing  material 
that  suffice  to  abundantly  seed  the  milk.     Soxhlet1  found 
that  the  addition  of  0.1  per  cent,  of  sour  milk  to  fresh 
milk  decreased  the  keeping  quality  of  the  latter  from  15— 
30  per  cent.;  the  addition  of  1.5  per  cent,  diminished  it 
80  per  cent.     Where  cans  are  not  well  cleaned  the  above 
amount  could  easily  be  added  to  the  milk  from  the  ma- 
terial that  adhered  to  the  walls  of  the  can. 

Through  negligence,  vessels  are  often  used  that  are 
either  unfit  or  are  in  an  improper  condition  for  handling 
milk.  A  rusty  milk- can  often  spoils  more  milk  than 
sufficient  to  purchase  a  new  vessel.  Wooden  pails  are 
no  longer  to  be  tolerated  in  a  well-regulated  dairy. 
Where  possible,  vessels  should  be  made  of  pressed  tin. 
If  joints  are  necessary,  they  should  be  well  flushed  with 
solder  so  that  they  may  be  easily  and  thoroughly  cleaned. 
In  much  of  the  cheap  tinware  that  is  now  to  be  found  on 

1Soxhlet,  Ber.  d  Wanderversammlung  bayer.  Landwirthe,  Oct., 
1894. 


Contamination  of  Milk.  27 

the  market,  there  is  altogether  too  thin  a  veneer  of  solder 
to  effectually  cover  the  joints. 

34.  Use  of  milk-cans  for  transporting"  factory  by- 
products. The  general  custom  of  using  the  milk-cans 
to  carry  back  to  the  farm  the  factory  by-products  (skim- 
milk  or  whey)  has  much  in  it  that  is  to  be  deprecated. 
These  by-products  are  generally  rich  in  bacterial  life, 


FIG.  6.  The  wrong  and  the  right  kind  of  a  milk-pail.  A,  the  ordinary  type  of 
pail  showing  sharp  angle  between  sides  and  bottom;  B,  the  same  properly 
flushed  with  solder  so  as  to  facilitate  thorough  cleaning.  The  lower  figure  rep- 
resents a  joint  as  ordinarily  made  in  tinware.  The  depression  a  affords  a  place 
of  refuge  for  bacteria  from  which  they  are  not  readily  dislodged.  This  open  joint 
should  be  filled  completely  with  solder. 

more  especially  where  the  closest  scrutiny  is  not  given  to 
the  daily  cleaning  of  the  vats  and  tanks.  Too  frequently 
the  cans  are  not  cleaned  immediately  upon  arrival  at  the 
farm,  so  that  the  conditions  are  favorable  for  rapid  fer- 
mentation. This  danger  can  be  entirely  obviated  by  a 
thorough  cleansing  process,  but  the  patron  who  is  shift- 
less in  regard  to  his  cans  will  generally  be  lax  in  his 
treatment  of  these  vessels,  and  thus  the  opportunity  for 
infection  is  present. 

Many  of  the  taints  that  bother  factories  are  directly 
traceable  to  such  a  cause.  A  few  dirty  patrons  will  thus 
seriously  infect  the  whole  supply. 


28  Dairy  Bacteriology. 

The  responsibility  of  this  defect  should,  however,  not 
be  laid  entirely  upon  the  shoulders  of  the  producer.  The 
factory  operator  should  see  that  the  refuse  material  does 
not  accumulate  in  the  waste  vats  from  day  to  day  and  be 
transformed  into  a  putrid  mass.  A  dirty  whey  tank  is 
not  an  especially  good  object  lesson  to  the  patron  to  keep 
his  cans  clean. 

It  is  possible  that  abnormal  fermentations  may  thus  be 
disseminated  from  one  farm  to  another  in  this  way. 

Suppose  there  appears  in  the  dairy  of  A  an  infectious 
milk  trouble,  such  as  bitter  milk.  This  milk  is  taken  to  the 
factory  and  passes  unnoticed  into  the  general  milk- supply. 
The  skim-milk  from  the  separator  is  of  course  infected 
with  the  germ,  and  if  conditions  favor  its  growth,  the 
whole  lot  soon  becomes  tainted.  If  this  waste  product  is 
returned  to  the  different  patrons  in  the  same  cans  that 
are  used  for  the  fresh  milk,  the  probabilities  are  strongly 
in  favor  of  some  of  the  cans  being  contaminated  and  thus 
infecting  the  milk- supply  of  other  patrons.  If  the  organ- 
ism is  endowed  with  spores  so  that  it  can  withstand  un- 
favorable treatment,  this  disease  may  spread  from  patron 
to  patron  simply  through  the  infection  of  the  vessels  that 
are  used  for  the  transportation  of  the  by-products. 

It  would  be  possible  to  obviate  any  trouble  arising  from 
this  source  if  a  separate  receptacle  was  used  for  this  pur- 
pose, but  the  objection  is  frequently  urged  that  this  is 
impractical,  yet  many  of  the  more  progressive  factories 
are  following  this  practice  with  excellent  results. 

35.  Effect  of  steaming1  milk-pails.  Even  where 
utensils  are  in  good  condition  and  well  cleaned,  the  germ 
content  of  the  milk  may  be  reduced,  and  therefore,  the 
keeping  quality  enhanced  by  a  brief  steaming  of  the  re- 
ceiving cans.  For  this  experiment1  two  cans  were  taken, 

1  Russell,  nth  Kept.  Wis.  Agr'l.  Expt.  Stat.,  p.  152,  1894. 


Contamination  of  Milk.  29 

one  of  which  had  been  cleaned  in  the  ordinary  way,  while 
the  other  was  sterilized  by  steaming.  Before  milking,  the 
iidder  of  animal  was  thoroughly  cleaned,  and  special  pre- 
cautions taken  to  avoid  raising  of  dust;  the  fore  milk 
was  also  rejected.  Milk  was  then  drawn  directly  into 
these  two  cans  and  bacterial  determinations  made: 

Number  of  bacteria  per  cc.  in  steamed  pail,  165. 

Number  of  bacteria  per  cc.  in  ordinary  pail,  4265. 

Time  before  milk  in  steamed  pail  soured,  28-g-  hours. 

Time  before  milk  in  ordinary  pail  soured,  23  hours. 

If  an  imperfectly  cleaned  pail  had  been  used  for  this 
purpose,  the  difference  in  souring  would  have  undoubt- 
edly been  more  apparent. 

To  illustrate  the  varying  germ  content  of  cans  cleaned 
in  different  ways,  Harrison1  rinsed  out  cans  with  100  cc. 
of  sterile  water,  and  then  determined  the  germ  content  of 
the  same.  The  following  data  represents  the  number  of 
bacteria  per  cc.  in  the  rinsing  water  from  cans  improperly 
cleaned  (series  A),  from  cans  washed  in  tepid  water  and 
then  scalded  —  the  usual  factory  method  —  ( series  B )  and 
from  cans  washed  in  tepid  water  and  steamed  for  five 
minutes  (series  C.). 

Effect  of  steaming  on  germ  content  of  cans. 

Series  A  (Dirty  cans) ..   238,525,  342,875,  215,400,  618,200,  806,320, 

510,270,   230,100,  610,510,  418,810,   317,250. 
Series  B  (Ordinary 

cleaning  method)...     89,320,     84,750,     26,800,     24,000,     38,400, 

76,800,     15,200,     13,080,     44,160,     93,400. 
Series  C  (Approved 

method) 1,170,       1,792,          890,          355.          416. 

36.  Cleaning1  dairy  utensils.  Milk  vessels  should 
never  be  allowed  to  become  dry  when  dirty,  for  dried 
particles  of  milk  residue  are  extremely  difficult  to  remove. 

1  Harrison,  22nd  Kept.  Ont.  Agr'l.  Coll.,  p.  113.  1896. 


30  Dairy  Bacteriology. 

In  cleaning  dairy  utensils  they  should  first  be  rinsed  in 
lukewarm  instead  of  hot  water,  so  as  to  remove  organic 
matter  without  coagulating  the  milk.  Then  wash  thor- 
oughly in  hot  water,  using  soap  or  weak  alkali.  A  borax 
solution  is  sometimes  recommended  for  cleaning  bottles. 
Strong  alkalies  should  not  be  used.  After  washing,  rinse 
thoroughly  in  .  clean  hot  water  ;  then  invert  over  a  steam 
jet  for  a  few  minutes.  A  momentary  application  of  hot 
or  even  boiling  water  is  insufficient  to  destroy  germ  life 
that  lurks  in  joints  of  vessels.  Live  steam  is  especially 
efficient  as  a  germ-  destroy  ing  agent.  If  steamed,  the 
cans  will  dry  more  quickly. 

It  is  not  often  that  steam  is  available  on  the  farm,  but 
under  such  conditions,  it  is  possible  to  acquire  practically 
the  same  results  by  using  boiling  water,  although  the 
length  of  exposure  must  be  increased. 

Not  only  should  the  greatest  care  be  paid  to  the  condi- 
tion of  the  cans  and  milk-pails,  but  all  dippers,  strainers, 

and  other  utensils  that  come 
in  contact  with  the  milk,  must 
be  kept  thoroughly  clean. 
Cloth  strainers,  unless  at- 
tended to,  are  objectionable, 
for  the  fine  mesh  of  the  cloth 
retains  so  much  moisture  that 
they  become  a  veritable  hot- 
bed of  bacterial  life,  unless 
they  are  daily  boiled  or 

FIG.  7.    Section  of  udder  showing    cf  aarn  g(J 
relation  of  milk-secreting  tissue  to 
milk  duct  (after  Thanhoffer).    a,  ex-          37.    Influence    Of    flPSt    OP 


secreting  tissue;  e,  sphincter  mus-    in   the  Contamination  of  milk, 

the    importance    of   which    is 

rarely  recognized,  comes  from  the  bacteria  that  gain  ac- 
cess to  the  milk,  by  mixing  the  first  or  fore  milk  with 


Contamination  of  Milk.  31 

the  remainder  of  the  milk.  Even  when  the  milking  is 
most  thoroughly  done,  there  remains  in  the  milk  ducts 
of  the  udder,  a  few  drops  of  milk  that  afford  sufficient 
nutriment  for  the  development  of  any  germs  that  may 
gain  access  through  the  opening  of  the  teat.  Accord- 
ing to  Gernhardt,  it  is  possible  that  they  may  penetrate 
the  udder  as  far  as  the  milk  cisterns  or  gland  tissue  itself, 
but  the  evidence  on  this  point  is  not  decisive.  The  rela- 
tively high  temperature  of  the  teat  facilitates  a  rapid 
growth. 

Under  these  conditions,  a  small  number  of  organisms 
are  able  to  increase  in  such  numbers  that  the  first  few 
spurts  of  milk  contain  many  more  than  those  which  are 
subsequently  drawn.  The  following  data  by  Harrison1 
strikingly  illustrates  this  point: 

Number  of  bacteria  per  cc.  in  milk. 

Foremilk 26,070,  25,630,  38,420,  18,110,  54,800,    32,700, 

43,520,  27,830,  18,500,  29,400,  45,630,    48,700. 
Milk  after  removal 

of  fore  milk 1,246,  1,150,  1,430,  3,420,  1,560,         890, 

2,575,  4,820,  3,270,  1,285,  1,350. 

If  the  fore  milk  is  received  in  a  separate  vessel  and 
kept  protected  from  the  air,  it  will  generally  be  noted 
that  it  sours  more  rapidly  than  the  remainder  of  the 
milk. 

As  a  rule  the  number  of  different  species  found  in  the 
fore  milk  is  usually  small,  not  more  than  one  or  two 
forms  being  present  at  any  time.  As  to  the  character 
of  these  forms  data  is  conflicting.  Harrison2  reports 
finding  peptonizing  bacteria  in  the  same,  and  Marshall3 
states  that  organisms  are  found  that  resist  pasteurizing, 

1  Harrison,  226!  Kept.  Ont.  Agr'l.  Coll.,  p.  108,  1896. 

M.  c.,  p.   108. 

8  Marshall,  Mich.  Expt.   Stat.,  Bull.  147,  p.  42. 


32  Dairy  Bacteriology. 

a  characteristic  not  usually  associated  with  the  lactic  acid 
class. 

Bolley1  in  thirty  experiments  found  twelve  out  of  six- 
teen species  to  belong  to  the  lactic  acid  class.  In  no 
case  were  gas- generating  species  present.  This  fact  is 
important  in  the  selection  of  a  milk  from  a  single  animal 
for  the  cultivation  of  a  starter. 

This  condition  represents  the  milk  of  perfectly  healthy 
animals;  where  the  udder  is  diseased,  as  in  the  case  of 
infectious  garget  or  inflammation,  bacteria  may  be  pres- 
ent in  much  larger  numbers  and  affect  the  milk  seriously 
for  food  purposes. 

Not  all  species  seem  to  be  able  to  maintain  themselves 
in  the  udder  even  if  they  should  be  introduced.  Dinwid- 
die2  injected  into  the  milk  cistern  a  lactic  acid  producing 
facultative  anaerobe  that  grew  luxuriantly  at  99°  F.  An 
examination  of  the  milk  several  times  afterwards  failed 
to  show  the  presence  of  this  organism  in  any  case, 
although  other  species  were  isolated.  • 

38.  Dirt  from  animal.  By  many  it  is  believed  that 
much  of  the  germ  life  that  gets  into  the  milk  comes  from 
the  food  of  the  animal.  For  this  reason  fermenting 
food- stuffs  of  any  kind  are  regarded  as  unfit  for  use. 
While  material  of  this  class  is  not  a  suitable  nutrient  for 
any  animal,  the  danger  to  the  milk  is  not  due  to  the  bac- 
teria ingested  with  the  food,  but  to  a  large  extent  to  those 
that  adhere  to  the  animal's  coat,  and  subsequently  fall 
into  the  milk.  Of  course  the  udder  may  be  infected  if 
the  animal  is  diseased  as  in  inflammation  (mammitis),3 

1  Assoc.  Ag.  Coll.  and  Expt.  Stat.,1895,   also  Cent.  f.  Bakt.  II  Abt. 
1:  795,  1895. 

Second  article,  Bull.  No.  21,  N.  D.  Expt.  Station. 

2  Dinwiddie,  Ark    Expt.  Stat,  Bull.  45,  p.  57. 

3  Guillebeau,  Landw.  Jahr.  d.  Schweiz,  1892,  p.  27. 


Contamination  of  Milk.  33 

or  tuberculosis,  in  which  case  bacteria  find  their  way  di- 
rectly into  the  milk.  Marshall1  succeeded  in  isolating 
direct  from  the  udder,  a  pure  culture  of  Nocard's  strep- 
tococcus, the  cause  of  infectious  udder  inflammation. 

The  hairy  coat  of  the  animal  offers  exceptional  facili- 
ties for  the  harboring  of  dust  and  dirt.  Cows  wading  in 
stagnant  pools  cover  the  udder  with  slime  and  dirt  that 
is  readily  dislodged  when  dried.  The  hairy  coat  is,  there- 
fore, extremely  rich  in  the  various  forms  of  bacterial  life 
that  are  derived  from  the  particles  of  excreta  that  stick 
to  the  flanks  and  under  parts  of  the  animal.  Where  peat 
is  used  for  bedding,  favorable  reports  are  made  as  to  its 
value. 

The  amount  of  actual  impurities  that  are  to  be  found 
in  milk,  even  after  it  is  strained,  will  surprise  the  casual 
observer,  although  it  should  be  noted  according  to 
Backhaus,2  that  about  one-half  of  fresh  manure  dis- 
solves in  milk  and  thus  does  not  appear  as  sediment. 
From  a  large  number  of  determinations  of  the  solid  im- 
purities found  in  the  market  milk  of  different  European 
cities,  Renk3  deduces  the  following  rule:  If  a  sample  of 
milk  shows  any  evidence  of  impurity  settling  on  a  trans- 
parent bottom  within  two  hours,  it  is  to  be  regarded  as 
containing  too  much  solid  impurities.  These  solid  par- 
ticles, composed  largely  of  manure  and  dirt,  are  always 
teeming  with  bacteria,  especially  with  putrefactive  and 
decomposition  organisms.  It  has  recently  been  estimated 
that  the  city  of  Berlin  consumes  daily  300  pounds  of  dirt 
and  filth  in  its  milk- supply. 

Not  only  is  the  number  of  bacteria  thus  introduced  into 
the  milk  very  considerable,  but  the  character  of  the  same 

1  Marshall,  Mich.  Expt.  Stat.,  Bull.  146,  p.  6. 
'Backhaus,  Milch  Ztg.,  26:  357,  1897. 
3  Renk,  Cent.  f.  Bakt.,  10:  193. 
3-B. 


34  Dairy  Bacteriology. 

is  a  question  of  even  greater  importance.  Those  species 
that  are  derived  primarily  from  manure  particles  are,  as 
a  rule,  the  peptonizing  or  digestive  species  that  cause  a 
decomposition  of  the  casein,  and  should,  therefore,  be 
avoided  if  possible. 

Improper  stable  conditions  greatly  favor  the  amount  of 
filth  that  may  adhere  to  the  animal.  The  more  highly 
nitrogeneous  feeding  that  is  practiced  at  present  pro- 
duces a  softer  manure  and  one  in  which  putrefactive  bac- 
teria are  much  more  likely  to  be  abundant. 

Wiithrich  and  Freudenreich1  have  studied  the  influ- 
ence of  feeding  on  the  bacterial  content  of  manure,  and 
they  find  a  markedly  higher  content  in  manure  where 
animals  are  given  dry  feed  than  where  kept  on  grass. 
The  character  of  the  manure,  however,  is  different,  it 
being  much  more  liquid  with  moist  than  dry  feed,  and 
therefore,  they  believe  more  likely  to  find  its  way  into  the 
milk.  They  found  as  many  as  375,000,000  bacteria  per 
gram  in  fresh  manure,  the  majority  of  which  consisted  of 
B.  coli  communis,  the  hay  bacillus,  and  other  species  able 
to  peptonize  the  casein. 

39.  Influence  of  tne  milker.  The  condition  of  the 
milker  is  by  no  means  an  unimportant  factor.  If  he  per- 
forms the  milking  in  the  dust- laden  garments  that  he 
has  worn  all  day,  he  himself  is  covered  with  particles 
that  are  readily  dislodged  when  he  comes  in  contact  with 
the  cow. 

Particular  attention  should  be  paid  to  the  hands  of  the 
milker.  The  habit  of  moistening  the  hands  with  a  few 
drops  of  milk  just  before  milking  is  to  be  deprecated 
from  every  standpoint,  but  especially  so,  when  consid- 
ered from  our  present  point  of  view.  After  having 

!Cent.  f.  Bakt.,  II  Abt.,  1:  873,  1895. 


^  OF  THB  r 

UNIVERSITY 


Contamination  of  Milk.  35 

washed  his  hands  in  clean  water,  a  pinch  of  vaseline  on 
his  hands  will  enable  him  to  obtain  a  firmer  grasp, 
and  at  the  same  time,  any  scales  or  dirt  rubbed  from  the 
teat  would  be  held  by  the  vaseline.  Its  healing  effect  on 
chapped  or  sore  teats  would  also  be  helpful.  Freuden- 
reich1  reports  some  experiments  in  which  the  germ  con- 
tent of  milk  was  reduced  from  several  thousand  to  200 
where  the  hands  were  well  rubbed  with  vaseline  before 
milking.  Where  the  best  of  conditions  are  carried  out 
it  is  worth  while  to  have  the  milker  clothed  in  a  suit 
kept  for  this  purpose,  especially  the  upper  portion  of  the 
body.  An  outer  garment  could  easily  be  slipped  over 
the  regular  working  clothes.  This  garment  should  be 
laundried  at  frequent  intervals. 

40.  Exclusion  of  dirt.  A  large  amount  of  filth  and 
dirt  can  be  prevented  from  falling  into  the  milk.  Card- 
ing the  udder  and  flanks  to  remove  the  loose  hairs  will 
remove  a  considerable  source  of  dirt.  So  long,  however, 
.as  the  coat  of  the  animal  is  dry,  dust  particles  with  their 
adherent  bacteria  are  readily  dislodged.  If  the  coat  of 
the  animal  is  moist,  this  deposition  can  be  almost  effect- 
ually prevented.  The  surface  should,  however,  not  be 
dripping  wet.  The  objection  has  been  urged  by  some 
that  washing  the  udder  starts  the  milk  secretion,  and  un- 
less the  animal  is  milked  at  once,  the  yield  of  milk  is 
diminished  thereby.  Eckles2  has  recently  reported  a 
number  of  experiments  from  which  he  concludes  that 
when  the  animal  is  accustomed  to  the  treatment,  no  no- 
ticeable effect  is  produced  either  in  amount  of  milk  or 
butter  fat. 

In  order  to  show  the  effect  of  dirt  and  dust,  the  experi- 
ment described  below  teaches  a  valuable  lesson.  A  cow 

ipreudenreich,  Die  Bakteriologie,  p.  30. 
*  Eckles,  Hoard's  Dairyman,  Aug.  8,  1898. 


36  Dairy  Bacteriology. 

that  had  been  pastured  in  a  meadow  was  partially  milked 
out  of  doors.  During  the  operation  a  covered  glass  dish 
containing  sterile  gelatin  was  exposed  for  sixty  seconds 
underneath  the  belly  of  the  cow  in  close  proximity  to  the 
milk  pail.  The  udder,  flank,  and  legs  of  the  cow  were 
then  thoroughly  cleaned  with  water,  and  all  of  the  pre- 
cautions referred  to  before  were  carried  out,  and  the  milk- 
ing then  resumed.  A  second  plate  was  then  exposed  in 
the  same  place  for  an  equal  length  of  time;  a  control 
also  being  made  at  the  same  time  at  a  distance  of  ten 
feet  from  the  animal  and  six  feet  from  the  ground  to  as- 
certain the  germ  contents  of  the  surrounding  air. 

From  this  experiment  the  following  instructive  data 
were  gathered.  Where  the  animal  was  milked  without 
any  special  precautions  being  taken,  there  were  3,250 
bacterial  germs  per  minute  deposited  on  an  area  equal  to 
the  exposed  top  of  a  ten- inch  milk  pail.  Where  the  cow 
received  the  precautionary  treatment  as  suggested  above, 
there  were  only  115  germs  per  minute  deposited  on  the 
same  area.  In  the  plate  that  was  exposed  to  the  sur- 
rounding air  at  some  distance  from  the  cow,  there  were 
sixty- five  bacteria.  This  indicates  that  a  large  number 
of  organisms  from  the  dry  coat  of  the  animal  can  be  kept 
out  of  milk  if  such  simple  precautions  as  these  are  in- 
stituted. 

Another  method  of  exclusion  is  to  use  a  milk  pail  hav- 
ing a  partially  closed  top  that  will  prevent  the  introduc- 
tion of  a  large  part  of  the  dirt. 

Still  another  method  that  has  much  to  recommend  it, 
is  passing  the  milk  through  a  hand  separator  immedi- 
ately. This  not  only  removes  nearly  all  of  the  suspended 
particles  oi;  foreign  matter,  such  as  dirt  and  filth  of  vari- 
ous kinds,1  but  it  eliminates  a  large  part  of  the  bacteria 

1  Backhaus  (Milch  Ztg.,  26:  358,  1897)  found  that  95.6  per  cent,  of 
impurities  were  removed  by  centrifugal  separation. 


Contamination  of  Milk.  37 

with  the  same,  if  it  is  done  immediately  after  the  milk  is 
drawn.  The  only  objection  to  this  process  is  that  the 
cream  does  not  rise  so  thoroughly  as  where  it  is  not  cen- 
trifuged,  but  the  other  advantages  where  an  especially 
clean  milk  is  wanted  more  than  compensate  for  this 
defect. 

41.  Influence  of  air.  It  is  impossible  to  separate 
the  influence  of  the  air  entirely  from  that  of  the  animal, 
as  the  dust  particles  from  the  coat  of  the  animal  must 
of  necessity  pass  through  the  air. 


FIG.  8.— Effect  of  contaminated  air.  Each  spot  on  this  surface  represents 
a  developing  colony  that  has  grown  from  a  germ  that  was  deposited  on  surface 
of  a  sterile  gelatin  plate  (3  inches  in  diam.)  in  30  seconds.  This  exposure  was 
made  at  time  the  cows  were  fed. 

Germ  life  cannot  develop  in  the  air,  but  in  a  dried  con- 
dition, organisms  retain  their  vitality  for  long  periods  of 
time.  The  use  of  dry  fodder,  the  bedding  of  animals 
with  straw  adds  greatly  to  the  amount  of  dust  particles, 
and  consequently  the  germ  life  floating  in  the  air,  as 
seen  in  fig.  8.  Taints  in  milk  have  frequently  been 
traced  to  infection  arising  from  this  source. 


38  Dairy  Bacteriology. 

While  the  stable  air  cannot  be  freed  entirely  from  dust, 
much  can  be  done  with  a  little  forethought.  Feeding 
before  milking  adds  materially  to  the  germ  content,  espe- 
cially if  feed  is  of  dry  character.  If  moistened  feed  is 
given  just  before  or  during  milking,  the  same  objection 
does  not  inure.  Harrison1  gives  some  striking  results 
on  this  point.  The  numbers  cited  indicate  the  num- 
ber of  bacteria  that  were  deposited  per  minute  on  a 
surface  equal  to  that  of  a  twelve- inch  milk  pail.  In 
Series  A,  the  exposure  was  made  during  bedding;  in  B, 
this  operation  was  performed  an  hour  before. 

Influence  of  dusty  air  on  germ  life. 

Series  A 16,000,         13,536,         12,216,         12,890,         15,340, 

19,200,        23,400,         27,342,         42,750,         18,730. 
Series  B 483,  610,  820,  715,  1,880, 

.      2,112,          1,650,  990,  1,342,          2,370. 

These  results  indicate  that  the  bacteria  are  for  the 
most  part  attached  to  particles  of  considerable  weight  as 
they  settle  readily  to  the  floor. 

42.  The  relative  importance  of  foregoing  factors. 
The  relative  importance  of  these  various  factors  differs 
so  much  in  different  cases  that  110  uniform  rule  can  be 
given  as  to  the  effect  of  their  presence  in  different  cases. 
In  those  dairies  where  no  especial  care  is  exercised  over  the 
methods  of  handling  the  milk,  the  factor  of  unclean 
utensils  and  filth  from  the  animal  are  usually  the  great- 
est. Where  a  careful  supervision  is  given  to  these  details, 
the  influence  of  the  fore  milk  is  of  first  importance.  The 
effect  of  these  various  contaminating  factors  can  be 
largely  minimized,  and  in  some  instances  eliminated  with- 
out great  difficulty. 

A  goodly  number  of  the  bacteria  present  in  the  fore 
milk  can  be  prevented  from  gaining  access  to  the  milk  by 
rejecting  the  first  few  streams  that  are  milked  from  each 

1  Harrison,  22d  Kept.  Ont.  Ag.  Coll.,  1896,  p.  111. 


Contamination  of  Milk.  39 

teat,  although  this  factor  can  be  controlled  to  a  less  de- 
gree than  any  of  the  others.  The  actual  loss  occasioned 
in  doing  this  is  very  slight,  as  the  first  part  of  the  milk- 
ing is  very  poor  in  butter  fat. 

Dust  in  the  air  of  the  barn  is  a  minor  factor,  yet  this 
can  be  materially  reduced  by  exercising  care  relative  to 
feeding  dry  fodder,  or  bedding  the  animals  just  previous 
to,  or  during  the  milking. 

The  number  of  germs  derived  from  the  animal  itself 
and  the  milker  can  be  largely  decreased  by  keeping  the 
animal  thoroughly  clean,  and  having  the  milker  milk  with 
clean  hands. 

The  effect  of  contamination  arising  from  imperfectly 
cleaned  milk  vessels  can  be  practically  excluded  by  ster- 
ilizing all  such  utensils  in  steam,  or  even  in  scalding 
water  for  a  short  time. 

To  determine  what  reduction  could  be  accomplished  by 
drawing  and  handling  the  milk  under  as  clean  conditions 
as  possible,  the  following  experiment  was  carried  out: 
The  udder  was  thoroughly  carded,  and  then  moistened; 
the  milk  received  in  steamed  pails,  the  fore  milk  being 
rejected. 

The  milk  from  a  cow  treated  in  this  way  contained  330 
bacteria  per  cc.,  while  that  of  a  mixed  herd  taken  under 
usual  conditions  was  15,500  bacteria  for  same  volume. 
This  carefully  handled  milk  kept  over  twenty  hours 
longer  at  room  temperatures  than  the  ordinary  product. 
Backhaus  estimates  that  the  germ  life  in  milk  can  be 
easily  reduced  to  Wire  of  its  original  number  by  using 
care  in  milking.  Methods  of  this  sort  must  be  instituted 
in  those  dairies  that  are  furnishing  the  highest  grade  of 
product.  Where  such  methods  are  in  vogue,  bacteria 
are  excluded  for  the  most  part  and  pasteurization  be- 
comes unnecessary.  Such  milks  are  frequently  known  as 


40  Dairy  Bacteriology. 

"sanitary"  or  "certified"  as  the  method  of  handling  the 
herd  is  often  nnder  the  control  of  ontside  parties  (phy 
sicians  or  health  board)  who  certify  to  the  regulations 
that  are  followed. 


FIG.  9.  Bacterial  content  of  milk  handled  in  ordinary  way.  Each  spot  rep- 
resents a  colony  growing  on  gelatin  plate.  Compare  with  fig.  10,  where  same 
quantity  of  milk  is  used  in  making  culture.  Over  15,000  bacteria  per  cc.  in  this 
milk. 

The  question  may  arise  as  to  the  necessity  of  these 
precautionary  measures  if  the  milk  is  to  be  used  for  or- 
dinary factory  purposes.  While  certain  bacteria  are 
essential  in  the  butter  and  cheese  industry  to  secure  a 
normal  and  characteristic  fermentation,  yet  it  is  better  to 
have  the  germ  life  reduced  to  the  lowest  numbers  than 
to  have  the  raw  milk  infected  through  slovenly  methods 
of  handling.  If  this  is  done,  the  maker  has  the  fer- 
mentation under  his  control,  and  by  the  addition  of  a 
starter  which  he  can  choose,  he  can  vary  the  character  of 
the  product  to  suit  the  demands  of  the  trade,  which  he 
cannot  do,  if  the  raw  milk  is  brought  to  him  in  a  dirty 
condition,  and  in  an  advanced  stage  of  fermentation. 


Contamination  of  Milk.  41 

Dairymen  have  learned  many  of  these  lessons  in  the 
severe  school  of  experience,  but  the  reason  for  the  same 
is  so  palpably  plain  in  the  light  of  bacteriological  explana- 
tion, that  further  discussion  would  seem  unnecessary.  It 


FIG.  10.  Bacterial  content  of  milk  drawn  with  care.  Diminished  germ  con- 
tent is  shown  by  smaller  number  of  colonies  (330  bacteria  per  cc.).  Compare 
this  culture  with  that  shown  in  fig.  9. 

remains  to  be  seen  whether  the  words  of  the  eminent 
German  authority,  Prof.  Fleischmann  will  much  longer 
prove  true,  when  he  says  that  "all  the  results  of  sci- 
entific investigation  which  have  found  such  great  prac- 
tical application  in  the  treatment  of  disease,  in  disinfec- 
tion and  in  the  preservation  of  various  products,  are 
almost  entirely  ignored  in  milking. " 

43.  Effect  of  temperature  on  bacterial  growth. 
After  milk  is  once  seeded  with  bacterial  life,  no  one  factor 
exerts  so  potent  an  effect  upon  the  rate  of  change  that 
takes  place  as  that  of  temperature,  for  the  rapidity  of 
bacterial  growth  in  milk  is  determined  mainly  by  this 
factor  as  is  seen  in  fig.  11. 


42  Dairy  Bacteriology. 

Although  different  species  vary  in  their  rate  of  devel- 
opment, yet  moderately  warm"  temperatures  from  75°  to 
90°  F.,  encourage  rapid  growth.  Unless  the  milk  is 


PROGENY  Or  A 

SINGLE  GERM 
IN  TWELVE:  HOURS 


FIG.  11— Showing  the  effect  of  cooling  milk  on  the  growth  of  bacteria.    The 
beneficial  results  of  early  chilling  are  readily  apparent. 

quickly  deprived  of  its  original  heat,  the  rate  of  the  fer- 
mentative changes  will  be  much  increased  as  is  shown  in 
following  results  obtained  by  Cnopf  and  Escherich : 

Rate  of  growth  of  single  germ 

2  hrs.  3  hrs.  4  hrs.  5  hrs.  6  hrs. 

54°  F 4    6      8     26    435 

97°  F 23   60    215    1830   3800 

If  a  can  of  milk  is  allowed  to  cool  naturally,  it  will  take 
several  hours  before  it  reaches  the  temperature  of  the 
surrounding  air.  During  this  time,  the  organisms  in 
the  fore  milk  are  continuing  their  rapid  growth,  while 
those  forms  that  come  from  dust,  and  are  presumably 
in  a  latent  state  on  account  of  their  condition,  awake 
from  their  lethargy  under  the  influence  of  these  favora- 
ble surroundings.  If  bacteria  once  gain  an  entrance  and 
begin  to  germinate,  a  considerably  lower  temperature  is 
required  to  successfully  check  development  than  to  hold 


Contamination  of  Milk.  43 

latent  organisms,  like  spores,  in  a  condition  where  germi- 
nation will  not  occur. 

To  hasten  this  lowering  of  temperature,  artifical  cool- 
ing is  a  necessity.  Coolers  using  cold  running  water  or 
ice  water  are  efficient  in  reducing  the  temperature  to  a 
point  where  development  is  much  checked. 

44.  Tainted  milks.     Not  all  taints  in  milk  can  be 
traced  to  the  development  of  bacterial  causes.     In  many 
cases   they   are  produced  by  the   direct   absorption  of 
odors  in  a  purely  physical  way  or  to  some  unusual  condi- 
tion of  the  system  of  the  animal.     In  some  cases  with 
animals  old  in  lactation,  the  milk  becomes  abnormal  in 
that  the  cream  does  not  rise  readily.     Such  milks  are 
known  as   "lazyn   or  "dead"   milks.      Then  again,  the 
dairyman    may    experience  difficulty  in    churning,   but 
more  often  these  difficulties  are  attributable  to  the  non- 
performance  of  important  manufacturing  details  rather 
than  to  a  perverted  condition  of  the  milk  itself.     Milk  is 
very  prone  to  absorb  volatile  odors,  the  fat  especially 
having  a  great  affinity  for  many  of  these  substances. 

45.  Direct  absorption  from  animal.    Odors  of  this 
sort  may  be  absorbed  by  the  milk  previous  to  or  after 
the  milking  has  occurred.    The  peculiar  "cowy"  or  "an- 
imal odor7 '  of  fresh  milk  is  an  illustration  of  an  inherent 
peculiarity  of  the  milk.     If  certain  strong  flavored  sub- 
stances as  onions,  certain  root  crops,  or  other  vegetables- 
are  consumed  by  the  cow,  the  odor-yielding  substances 
in  the  same  may  reappear  in  the  milk,  especially  if  the 
animal  partakes  of  these  a  short  time  before  milking. 
According  to  Kober  and  Busey,  the  milk  of  swill-fed  cows 
has   a  peculiar   taste,  and  is  said  to  produce  a  highly 
acid   urine   and   eczema.     Brewers'    grains   and   distil- 
lery slops  when  fed  in  large  quantities   frequently  in- 
duce an  abnormal  chemical  reaction  of  the  milk.     Taints 


44  Dairy  Bacteriology. 

of  this  sort  can  easily  be  prevented  by  taking  care  that 
dairy  stock  is  not  fed  such  feed. 

46.  Straining"  milk  in  the  barn.     For  convenience, 
milk  is  often  strained  in  the  barn,  but  the  danger  of 
tainting  the  same  where  this  is  done  is  so  great  that  the 
custom  is  not  to  be  recommended.     In  straining  the  milk 
so  much  surface  is  exposed  to  the  air  that  considerable 
bacterial  infection  can  occur,  unless  this  is  carried  out  in 
a  room  free  from  all  dust  particles.     Where  the  process 
is  done  in  the  barn,  the  can  with  its  strainer  is  often  left 
uncovered.     Under  these  conditions,  a  constant  deposi- 
tion of  germ  life  is  taking  place,  and  with  every  pailful 
of  milk  strained,  this  is  washed  through  the  meshes  of 
the  strainer  into  the  milk  below. 

47.  Absorption  of  odors  in  fresh  milk.    Not  only 
is  straining  in  the  barn  to  be  deprecated  from  the  above 
standpoint,  but  the  possibility  of  direct  physical  absorp- 
tion of  existing  taints  in  the  stable  should  warn  one 
against  this  custom.     It  is  a  commonly  accepted  idea  that 
milk  evolves  odors  and  cannot  absorb  them  so  long  as  it 
is  warmer  than  the  surrounding  air,  but  from  experi- 
mental evidence, l  the  writer  has  definitely  shown  that  the 
direct  absorption  of  odors  takes  place  much  more  rapidly 
when  the  milk  is  warm  than  when  cold,  although  under 
either  condition,  it  absorbs  volatile  substances  with  con- 
siderable avidity.     In  testing  this,  fresh  milk  was  ex- 
posed  to  an   atmosphere    impregnated    with   odors    of 
various  essential  oils  and  other  peculiar  aromatic  sub- 
stance, whose  odors  could  be  readily  identified.     Under 
these  conditions  the  cooled  milk  was  tainted  very  much 
less  than  the  milk  at  body  temperatures,  even  where  the 
exposure  was  for  a  half  hour. 


Russell,  15th  Kept.  Wis.  Expt.  Stat.,  p.  104,  1898. 


Contamination  of  Milk.  45 

48.  Aeration.     Practical  experience  has  long  demon- 
strated the  advantage  of  aerating  the  milk  as  soon  after 
milking  as  possible.     This  is  accomplished  in  a  variety 
of  ways.     In  some  cases,  air  is  forced  into  the  milk;  in 
others,  the  milk  is  allowed  to  distribute  itself  in  a  thin 
sheet  over  a  broad  surface  and  fall  some  distance  so  that 
it  is  brought  intimately  in  contact  with  the  air.     The 
benefit  claimed  for  aeration  is  that  foul  odors  and  gases 
which  may  be  present  in  the  milk  are  thus  allowed  to 
escape  by  bringing  the  finely  divided  milk  into  contact 
with  the  air.    As  ordinarily  practiced,  aeration  is  usually 
combined  with  cooling,  and  it  is  noteworthy  that  the 
most  effective  aerators  are   those   that   cool   simultane- 
ously.    Under  these  conditions,  the  keeping  quality  of 
the  milk  is  increased,  but  where  milk  is  simply   aerated 
without  cooling,  no  material  benefit  in  keeping  quality 
is  observed.     A  satisfactory  scientific  explanation  of  the 
advantages  of  aeration  has  not  yet  been  made.    It  is  dif- 
ficult  to  see  how  the  process  can  have  any  effect  on  the 
bacterial  life  in  the  milk.     Its  influence,  undoubtedly,  is 
on  the  odors  directly  absorbed  by  the  milk. 

49.  Distinction  between  bacterial   and  non-bac- 
terial defects  in  milk.     In  fresh  milk  it  is  relatively 
easy  to  distinguish  between  taints  caused  by  the  opera- 
tion of  external  biological  forces  and  those  due  to  direct 
absorption. 

If  the  taint  grows  more  pronounced  as  the  age  of  the 
milk  increases,  it  is  probably  due  to  the  living  organisms, 
as  the  taint- producing  bacteria  usually  gain  an  entrance 
after  the  milk  is  drawn  from  the  cow,  and  require  a  cer- 
tain period  of  incubation  before  undesirable  products  are 
formed.  Taints  due  to  errors  in  feeding  are  more  pro- 
nounced when  the  milk  is  first  drawn. 


46  Dairy  Bacteriology. 

The  absorption  of  flavors  by  old  milk  is  sometimes 
puzzling  to  distinguish  from  a  bacterial  trouble,  but  if 
the  surroundings  are  closely  noted,  the  cause  can  usually 
be  traced  to  delayed  absorption  of  bad  odors  from  the 
room  where  milk  is  stored. 

If  the  trouble  is  of  bacterial  origin,  it  can  frequently 
be  detected  by  transferring  a  small  quantity  of  the  sus- 
pected milk  to  a  fresh  lot,  preferably  that  which  has  first 
been  boiled.  If  due  to  a  living  organism,  the  abnormal 
fermentation  will  be  propagated  and  so  reproduce  the 
difficulty  in  the  infected  milk. 

B.     INFECTION  OF  MILK  IN  THE  FACTORIES. 

50.  Treatment  of  milk  in  factories.  Where  milk 
is  taken  to  a  creamery  or  cheese  factory,  the  method  of 
treatment  often  modifies  the  bacterial  life  of  the  milk  to 
-a  marked  degree. 

Butter  and  cheese-making  both  require  the  presence  of 
bacteria,  but  the  germ  life  must  be  of  the  right  sort  to 
be  beneficial  in  its  action.  For  this  reason  the  maker 
wishes  the  milk  to  be  as  free  as  possible  from  all  contam- 
inating influences.  If  the  foregoing  suggestions  are  in- 
telligently followed,  the  raw  material  will  be  turned  over 
to  the  factory  in  prime  condition ;  but  the  rational  treat- 
ment must  be  continued  here  if  the  best  of  products  is  to 
be  expected.  Numerous  influences  are  at  work  in  the 
factory  that  add  their  mite  to  the  milk  in  different  stages 
of  its  manufacture,  and  ultimately  affect  in  a  serious 
way  the  final  product.  Of  course,  very  much  depends 
upon  the  proper  observance  of  physical  conditions,  but 
-a  brief  outline  of  some  of  the  possible  dangers  will  also 
be  pertinent. 

The  cardinal  precept  in  the  factory  as  well  as  on  the 
farm  should  be  cleanliness.  Cleanliness  here  must  not 


Contamination  of  Milk.  47 

be  taken  to  mean  a  mere  absence  of  dirt  and  filth,  but  all 
utensils  that  come  in  actual  contact  with  the  milk  should 
be  rendered  as  germ-free  as  possible. 

From  the  time  that  the  milk  enters  the  weigh- can  until 
the  butter  is  in  the  tub,  and  the  cheese  on  the  curing- 
shelf,  it  should  be  remembered  that  many  opportunities 
for  infection  are  always  present. 

51.  Factory  utensils.     In  the  factory,  part  of  the 
same  set  of  factors  of  infection  are  at  work  as  are  found 
in  the  farm  dairy.     The  condition  of  factory  utensils  is 
always  a  point  of  prime  importance.     Where  steam  is 
accessible,  as  it  is  in  the  majority  of  cases,  there  is  no 
excuse  for  uncleanliness   of    any   sort,    as   most   pieces 
can  be  steamed  directly.     Open  vats  can  be  thoroughly 
scalded  if  covered  with  a  heavy  canvas  cloth.     Separator 
bowls,  churns,  cans,  and  dippers  should  always  receive  a 
daily  treatment.     The  rational  nature  of  these  methods 
is  to  be  seen  in  those  cases  where  the  same  utensil,  say 
a  dipper,  is  employed  indiscriminately  in  handling  all 
kinds  of    dairy  products.     All  cans  with   rusty  seams 
should  be  discarded.     Permit  no  vat  to  be  repaired  by 
putting  in  a  false  covering  over  the  old  one.     If  a  minute 
leak  is  established,  such  places  become  a  harbor  of  refuge 
for  all  kinds  of  putrefactive  organisms .     In  a  number  of 
cases  ill- smelling  factory  odors  have  been  traced  to  such 
a  cause. 

52.  Infection  from  air.     The  influence  of  the  air 
on  the  germ  content  of  the  milk  is,  as  a  rule,  overesti- 
mated.    If  the  air  is  quiet,  and  free  from  dust,  the  amount 
of  germ  life  in  the  same  is  not  relatively  large.     In  a 
creamery  or  factory,  infection  from  this  source  ought  to 
be   much  reduced,   for  the  reason  that  the  floors  and 
walls  are,  as  a  rule,  quite  damp,  and  hence  germ  life 
cannot  easily  be  dislodged.     The  majority  of  organisms 


48  Dairy  Bacteriology. 

found  under  such  conditions  comes  from  the  person  of 
the  operators  and  attendants.  Any  infection  can  easily 
be  prevented  by  having  the  ripening  cream- vats  covered 
with  a  canvas  cloth.  The  clothing  of  the  operator  should 
be  different  from  the  ordinary  wearing- apparel.  If  made 
of  white  duck,  the  presence  of  dirt  is  more  quickly  rec- 
ognized, and  greater  care  will  therefore  be  taken  than  if 
ordinary  clothes  are  worn. 

The  surroundings  of  the  factory  have  much  to  do  with 
the  danger  of  germ  infection.  Many  factories  are  poorly 
constructed  and  the  drainage  is  poor  so  that  filth  and 
slime  collect  about  and  especially  under  the  factory. 
The  emanations  from  these  give  the  peculiar  "factory 
odor"  that  indicates  fermenting  matter.  Not  only  are 
these  odors  absorbed  directly,  but  germ  life  from  the 
same  finds  its  way  into  the  milk,  contaminating  the 
same.  Connell1  has  recently  reported  a  serious  defect 
in  cheese  that  was  traced  to  germ  infection  from  defec- 
tive factory  drains.  According  to  Robertson,  sometimes 
it  becomes  necessary  to  remove  cheese  factories  to  new 
locations  before  the  bad  conditions  can  be  controlled. 

53.  Water  and  ice-supply  of  factories.  These  sup- 
plies should  be  carefully  controlled.  Water,  and  to  a 
less  extent  ice,  always  contain  bacteria  in  varying  num- 
bers, so  that  it  is  possible  that  undesirable  organisms 
may  be  introduced  from  this  source.  Most  creameries 
derive  their  water-supply  from  private  wells,  and  in  using 
these,  care  should  be  taken  that  they  are  arranged  so  as 
not  to  receive  any  surface  drainage.  A  deep  well  from 
which  the  water  is  used  in  large  quantities,  if  properly 
arranged,  will  contain  the  minimum  number  of  germs  as 
the  ground  water  is  practically  free  from  them. 


1Connell,  Kept,  of  Com.  of  Agric.,  part  xvi,  p.  15,  1897. 


Contamination  of  Milk.  49 

Harrison1  has  recently  traced  an  off- flavor  in  cheese 
in  a  Canadian  factory  to  an  infection  arising  from  the 
water-supply.  He  found  the  same  germ  in  both  water 
and  cheese  and  by  inoculating  a  culture  into  pasteurized 
milk  succeeded  in  producing  the  undesirable  flavor. 
Milk  can  easily  be  infected  through  using  such  water  for 
washing  utensils.  Some  well  waters  containing  iron  in 
solution  are  a  source  of  trouble  in  factories  on  account 
of  the  development  of  iron  bacteria  that  cause  the  solu- 
ble iron  to  be  precipitated  in  the  form  of  rusty  flakes  of 
ferric  oxid. 

In  ice  the  majority  of  organisms  (60-90%)  are  de- 
stroyed, but  enough  remain,  so  that  if  ice  is  secured 
from  a  highly  polluted  source,  the  living  germ  life  in  it 
may  be  an  element  of  danger.  It  is  not  considered  ad- 
visable to  add  ice  directly  to  milk  or  cream,  but  as  stor- 
age vessels  are  indiscriminately  used  to  contain  either 
ice  or  milk,  the  possibility  of  infection  should  be  kept 
in  mind. 

54.  Numbers  of  bacteria  in  milk.  The  germ  con- 
tent of  milk  varies  so  greatly  that  unless  the  conditions 
are  all  known,  it  is  impossible  to  foretell  what  may  be 
found  therein.  An  examination  of  milk  will  often  reveal 
a  difference  in  numbers,  ranging  from  a  few  score  of 
germs  to  hundreds  of  millions  per  cc.  The  presence  of 
such  a  varying  number  is  dependent  upon  certain  factors, 
as  the  age  of  the  milk,  the  care  taken  during  the  milking, 
and  also  the  way  in  which  it  has  been  handled  since  that 
time.  Disregarding  milk  of  different  ages,  the  number 
of  germs  present  in  any  sample  bears  a  general  relation 
to  the  amount  of  dirt  and  filth  with  which  it  has  come  in 
contact  since  it  was  drawn  from  the  cow.  Bacteria  and 

1  Harrison,  Hoard's  Dairyman,  Mch.  4,  1898. 
4-B. 


50  Dairy  Bacteriology. 

filth  of  all  kinds  are  so  intimately  associated  with  each 
other  that  the  presence  of  one  rightly  presupposes  that 
of  the  other. 

As  to  the  numerical  bacterial  content  of  any  milk, 
there  is  such  a  wide  variation  under  different  conditions 
that  figures  are  of  little  worth.  No  exact  relation  can  be 
maintained  between  number  of  bacteria  in  milk  and  the 
development  of  fermentative  products. 

The  studies  by  different  observers  have  been  carried 
on  under  such  diverse  conditions  that  no  comparison  of 
the  results  can  well  be  made.  Under  American  conditions 
but  little  work  has  been  done  in  this  direction,  yet  milk 
as  it  is  sold  here  to  the  consumer  usually  contains  less 
bacteria  than  that  retailed  in  European  cities,  although  as 
Conn  has  pointed  out,  it  is  materially  older.  As  he  in- 
timates this  fact  is  explained  by  the  relatively  free  use  of 
ice  in  this  country.  A  few  determinations  of  the  bac- 
terial contents  of  European  milks  that  have  been  analysed 
biologically  will  illustrate  this  point. 

Renk1  found  in  Halle  milk- supply  6-30,000,000  germs 
percc.;  Cnopf2in  Munich  milk- supply  200, 000-6, 000,- 
000  per  cc.;  Uhl3  in  Giessen  milk  83,000-170,000,000 
per  cc. ;  Clauss4  in  Wurzburg  222,000-23,000,000  per  cc. ; 
Bujwid  in  Warsaw  an  average  of  4,000,000  per  cc.  and 
Knochensteirn5  in  Dorpat  25,000,000  per  cc. 

Sedgwick  and  Batchelder6  report  fifty- seven  samples 
of  Boston  milk  as  containing  from  30,000-4,220,000  per 
cc.  In  the  country,  they  found  in  the  milk  fresh  from 


iRenk,  Cent.  f.  Bakt.,  10:  193. 
5  Cnopf,  Ibid.  6:  553. 

3  Uhl,  Zeit.  f.  Hyg.,  12:  475  (1892). 

4  Clauss,  Diss.  Wurzburg,  1889. 

5  Knochensteirn,  Chem.  Cent.,  11:  62. 

6  Sedgwick  &  Batchelder,  Boston  Med.  Surg.  Journ.,  Jan.  14,  1892. 


Contamination  of  Milk.  51 

the  cow  30,000  and  in  the  milk  as  used  on  the  table, 
about  70,000  organisms  per  cc.  Loveland  and  Watson1 
found  in  the  supply  of  Middletown,  Conn.,  from  11,000 
to  85,500,000  per  cc.  McClatchie2  found  in  Los  Angeles, 
Cal.,  supply,  an  average  of  11,700  per  cc.  in  twenty-six 
trials.  In  my  experience,  the  mixed  milk  of  a  herd  that 
is  kept  with  any  reasonable  degree  of  cleanliness,  if  ex- 
amined immediately  after  it  is  milked,  usually  will  not 
contain  more  than  5—20,000  germs  per  cc.  The  number 
present  in  any  milk  is  due  to  the  influence  of  so  many 
factors  that  it  is  practically  impossible  to  establish  any 
number  as  a  normal,  although  Bitter3  sets  50,000  germs 
per  cc.  as  a  maximum  limit  for  a  milk  intended  for  hu- 
man food.  The  milk  as  delivered  by  the  milkmen  to 
their  private  customers  in  the  city  of  Madison,  Wis., 
ranges  from  15,000-2,000,000  organisms  per  cc.,  varying 
mainly  with  the  season  of  the  year. 

The  presence  of  such  large  numbers  in  a  food  product 
need  not  necessarily  occasion  alarm  from  a  hygienic  stand- 
point, although  it  is  quite  certain  that  putrefactive  forms 
may  have  an  irritating  effect  upon  a  deranged  digestive 
tract,  and  thus  produce  intestinal  disturbances,  especially 
with  infants  during  the  summer  months. 

If  we  compare  the  bacterial  flora  of  milk  with  that  of 
sewage,  a  fluid  that  is  popularly  and  rightly  supposed  to 
be  teeming  with  germ  life,  it  will  almost  always  be  ob- 
served that  milk  when  it  is  consumed,  is  richer  in  bacte- 
ria by  far  than  the  sewage  of  our  large  cities.  Sedgwick4 
found  that  the  sewage  of  Lawrence,  Mass.,  contained  at 

1  Loveland  and  Watson,  7th  Report,  Storrs  Sta.  (Conn.),  1894,  p,  72. 

2  McClatchie,  Bull.  3,  Agr.  Expt.  Stat.  (So.  Cal.  Acad.  Sc.),  Aug. 
1897. 

3  Bitter,  Zeit.  f.  Hyg\,  8:  240. 

4  Sedgwick,  Kept.  Mass.  Bd.  Health,  1890,  p.  60. 


52  Dairy  Bacteriology. 

the  lowest,  100,000  germs,  while  the  maximum  number 
was  less  than  4,000,000  per  cc.  This  range  in  numbers 
is  much  less  than  is  usually  found  in  the  milk- supply  of 
our  large  cities. 

55.  Kinds  of  bacteria  in  milk.  The  number  of  bac- 
teria in  milk  is  not  of  so  much  consequence  as  the  kind 
present.  While  milk  may  contain  forms  that  are  inju- 
rious to  man,  still  the  great  majority  of  them  have  no 
apparent  effect  on  human  health.  In  their  effect  on 
milk,  the  case  is  much  different.  Roughly,  we  may  di- 
vide them  into  three  classes,  depending  upon  their  action 
in  milk. 

1.  Bacteria  that  exert  no  appreciable  effect  in  milk. 

2.  Bacteria  that  are  beneficial  by  reason  of  the  products 
which  they  form. 

3.  Bacteria  that  are  injurious  on  account  of  the  effect 
which  they  produce  in  milk. 

A  suprisingly  large  number  of  bacteria  that  are  found 
in  milk  belong  to  the  first  class.  Undoubtedly  they  af- 
fect the  chemical  characteristics  of  the  milk  somewhat, 
but  not  to  the  extent  that  it  becomes  physically  percep- 
tible. 

Those  species  that  are  concerned  in  the  production  of 
proper  flavor  and  aroma  in  butter,  and  which  are  also 
concerned  in  the  development  of  acid  and  possibly  asso- 
ciated with  formation  of  cheese  flavor  represent  the  sec- 
ond type.  Many  of  these  organisms  are  lactic  acid  pro- 
ducing, but  in  addition  to  these,  some  of  the  casein 
ferments  are  also  associated  with  aroma  production  in 
butter. 

The  third  class  includes  those  species  that  are  able  to 
produce  deleterious  and  undesirable  flavors  in  milk  and 
milk  products.  The  majority  of  the  abnormal  fermenta- 
tions of  milk  referred  to  in  this  chapter  come  under  this 


Contamination  of  Milk.  53 

head.  Most  of  these  gain  access  to  the  milk  through 
slovenly  and  careless  methods  of  handling.  Those  spe- 
cies associated  with  animal  excreta  are  particularly  dan- 
gerous. The  number  of  different  kinds  that  have  been 
found  in  milk  is  quite  considerable,  something  over  200 
species  having  been  described  more  or  less  thoroughly . 
In  all  probability,  however,  many  of  these  forms  will  be 
found  to  be  identical  when  they  are  subjected  to  critical 
study. 


CHAPTER  V. 
MILK  FERMENTATIONS  AND  THEIR  TREATMENT. 

56.  Classification  of  milk  fermentations.    If  any 

sample  of  milk  is  allowed  to  stand  at  any  ordinary  tem- 
perature for  several  days,  a  profound  physical  and  chemi- 
cal change  inevitably  takes  place.  As  a  rule  the  milk  will 
sour.  In  this  process,  certain  acids  are  formed  at  the 
expense  of  the  milk-sugar.  If  it  is  still  allowed  to  stand 
for  some  time  after  it  has  become  thoroughly  sour,  a 
series  of  subsequent  changes  usually  occur.  Often  it  will 
evolve  foul- smelling  gases  and  undergo  putrefactive 
changes.  Sometimes,  however,  other  ferment  organisms 
gain  the  ascendency  over  the  ordinary  souring  process, 
in  which  case  the  fermentation  is  said  to  be  abnormal. 
These  peculiar  changes  are  the  cause  of  various  taints  in 
milk.  They  are  of  great  importance  because  a  small 
quantity  of  milk  tainted  in  this  way  is  liable  to  infect  a 
much  larger  quantity  and  spoil  the  same  through  the 
rapid  development  of  the  obnoxious  organism.  In  some 
cases,  the  abnormal  condition  may  not  become  marked 
until  the  milk  is  made  up  into  some  other  product  as 
butter  or  cheese.  The  difficulties  that  occur  under  these 
conditions  will  be  discussed  under  their  appropriate  heads. 
It  is  impossible  in  the  present  state  of  our  knowledge 
to  classify  these  fermentations  in  any  other  than  a  most 
provisional  way.  While  our  knowledge  of  some  of  them 
from  a  bacteriological  standpoint  is  fairly  complete,  so 
many,  as  yet,  have  been  studied  only  superficially  that  we 
are  not  in  a  position  to  classify  them  as  we  would  other 

chemical  changes. 

[54] 


Milk  Fermentations.  55 

The  various  fermentations  may  be  grouped  according 
to  the  substances  in  the  milk  upon  which  they  chiefly 
act,  such  as  those  affecting  milk-sugar,  or  casein.  This 
arrangement  is  followed  in  this  connection,  but  it  should 
be  borne  in  mind  that  many  organisms  act  upon  several 
of  the  milk  constituents,  and  hence,  it  is  difficult  to 
correctly  classify  the  same. 

Another  method  of  classification  is  based  upon  the 
products  manufactured  during  the  fermentation,  but  even 
here  confusion  is  liable  to  enter  to  some  extent,  because 
in  many  cases  there  are  several  distinct  substances  formed. 

Milk  is  such  a  complex  substance  that  the  changes  pro- 
duced by  a  single  germ  are  often  so  numerous  that  the 
processes  can  not  be  separated  in  their  reactions.  It  must 
be  remembered  then,  in  referring  to  the  different  types 
of  fermentations,  that  perhaps  a  distinct  by-product 
is  being  formed,  but  it  is  more  than  probable  that  there 
are  a  series  of  changes,  in  which  the  most  marked  decom- 
position process  is  alone  taken  into  consideration.  For 
example,  there  is  a  fermentation  classed  under  the  head 
of  the  butyric  changes,  a  decomposition  process  in  which 
butyric  acid  is  the  chief  product  formed,  but  this  may  be 
associated  with  an  alkaline  condition  of  the  milk  and  the 
production  of  a  bitter  substance  in  the  same.  Thus,  the 
subdivision  followed  here  will  of  necessity  be  imperfect 
and  occasional  instances  will  be  noted  where  some  changes 
in  milk  might  well  be.  described  under  several  heads. 

Some  of  these  fermentation  changes  occur  so  con- 
stantly that  they  are  to  be  regarded  as  normal  or  natural 
in  their  character.  Normal,  however,  only  in  the  sense 
that  the  bacteria  that  cause  them  are  so  widely  distributed 
that  the  change  is  inevitable  and  not  that  the  milk  itself 
would  undergo  any  of  these  changes  if  it  were  not  for 
the  presence  of  the  different  germs  that  bring  them 


56  Dairy  Bacteriology. 

about.  Of  such  a  nature  is  the  lactic  acid  fermentation, 
by  far  the  most  common  change  that  occurs  in  milk. 

57.  Souring1  of  milk.  Milk  naturally  undergoes  a 
change  known  as  souring,  if  allowed  to  stand  for  several 
days  at  ordinary  temperature.  This  is  due  to  the  forma- 
tion of  lactic  acid,  which  is  produced  by  the  decomposition 
of  the  milk-sugar.  While  this  change  is  wellnigh  uni- 
versal, it  does  not  occur  without  a  pre-existing  cause,  and 
that  is  the  presence  of  certain  living  bacterial  forms. 
These  organisms  develop  in  milk  with  great  rapidity,  and 
the  decomposition  changes  that  are  noted  in  souring  are 
due  to  the  by-products  of  their  development. 

The  milk-sugar  undergoes  fermentation,  the  chief  pro- 
duct being  lactic  acid,  although  various  other  by-pro- 
ducts as  other  organic  acids  (acetic,  formic,  and  succinic) , 
different  alcohols,  and  gaseous  products,  as  CO2,  H,  N, 
and  methane  (CH4)  are  produced  in  small  amounts. 

In  the  souring  of  milk,  the  formation  of  acid  does  not 
continue  until  the  sugar  is  all  exhausted.  When  the 
acidity  reaches  about  0.4% ,  milk  begins  to  taste  sour.  In- 
cipient curdling  takes  place  at  about  0.6% ,  and  soon  after 
this,  bacterial  development  is  checked  altogether.  The 
acid  formation  goes  on,  however,  until  from  0.8  to  1.0% 
is  reached.  The  amount  of  acid  formed  varies  consider- 
ably in  different  cases.1 

If  the  acid  produced  in  any  case  is  removed  by  neu- 
tralizing it  with  a  carbonate,  its  development  will  begin 
anew,  showing  that  it  is  suspended  by  the  inability  of 
the  organisms  to  grow  in  such  a  medium. 

Cream  never  contains  as  much  acid  as  milk  for  the 
reason  that  a  considerable  proportion  of  its  volume  is  oc- 
cupied by  the  butter-fat  which  is  not  subject  to  this  de- 
composition. 

1  Warrington,  Journ.  Chem.  Soc.,  53:  727,  1888. 


.Milk  Fermentations.  57 

It  is  a  wide- spread  belief  that  thunder-storms  cause  the 
premature  souring  of  milk.  Numerous  experiments  have 
been  conducted  along  these  lines,  and  the  general  con- 
clusion is  that  neither  the  electric  discharge  nor  shock 
due  to  thunder  exert  any  effect  on  the  development  of 
acid,  but  that  the  atmospheric  conditions  usually  incident 
to  a  thunder-storm  are  such  as  permit  a  more  rapid  bac- 
terial growth. 

The  lactic  acid  fermentation  is  produced  by  a  large 
number  of  different  kinds  of  bacteria,  although  in  the 
spontaneous  coagulation  of  milk,  it  is  now  believed  that 
a  very  widely  distributed  species  is  responsible  for  the 
most  of  it.  This  organism  was  first  described  in  a  com- 
plete manner  by  Hiippe1  and  called  by  him  B.  acidi  lac- 
tici.  Giinther  &  Thierf elder2  working  on  the  spontaneous 
souring  of  milk  in  the  neighborhood  of  Berlin  found 
what  they  think  is  the  same  germ.  Esten3,  in  this 
country,  studied  milks  from  thirty  different  localities  in 
New  England  and  the  Middle  States.  He  found  a  germ 
in  all  but  two  cases  that  agreed  in  general  with  Giinther 's 
description.  Dinwiddie4,  studying  the  same  question  in 
Arkansas,  arrives  at  the  same  conclusion.  This  pre- 
ponderance of  evidence  makes  it  quite  probable  that  there 
is  a  widely  distributed  germ  that  is  concerned  in  this 
change  although  there  are  numerous  other  forms5  that 
are  associated  with  this  type  of  decomposition.  Conn 
and  Aikman  refer  to  the  fact  that  over  100  species  are 
already  known.  It  is  fair  to  presume,  however,  that  a 
careful  comparative  study  of  these  would  show  that 

1  Huppe,  Mitt.  a.  d.  k.  Gesund.   Amte,  2:  309,  1884. 

2  Giinther  &  Thierfelder,  Arch.  f.  Hyg.,  25:  164. 
3Esten,  9th  Kept.  Storrs  Expt.  Stat.,  p.  44,  1896. 

4  Dinwiddie,  Ark.  Expt.  Stat.,  Bull.  45,  May,  1897. 
5Kayser,  Ann.  de  1'Inst.  Past.,  10:  737. 


58  Dairy  Bacteriology. 

simply  racial  differences  exist  in  many  cases,  and  there- 
fore, that  they  are  not  distinct  species. 

As  a  rule  this  class  of  bacteria  is  unable  to  liquefy 
gelatin  or  develop  spores.  On  account  of  this  latter 
characteristic  they  are  easily  destroyed  when  milk  is  pas- 
teurized or  heated  to  a  higher  temperature.  They  live  un- 
der aerobic  or  anaerobic  conditions,  many  of  them  being 
able  to  grow  in  either  environment.  According  to  Hoft1 
spontaneous  souring  occurs  more  rapidly  in  vessels  hav- 
ing a  small  surface  exposed  to  the  air,  indicating  that 
anaerobic  activity  was  more  pronounced. 

The  temperature  conditions  as  to  growth  vary  some- 
what with  different  species.  With  most  species  growth 
occurs  at  50°  F.,  but  appreciable  amounts  of  acid  are  not 
produced  until  a  higher  temperature  is  reached2  . 

While  the  souring  of  milk  is  a  very  wide-  spread  phe- 
nomenon, still  lactic  acid  germs  do  not  abound  every- 
where. Esten  finds  them  abundantly  in  the  milk-  ducts 
but  not  present  on  hay.  From  the  milk  dealer's  stand- 
point, this  fermentation  like  all  others,  is  undesirable,  as 
it  destroys  the  food  value  of  milk  for  direct  consump- 
tion. The  fermented  product,  sour  milk,  has  some  value 
for  cooking  purposes.  From  this  fermented  product, 
the  Armenians  make  a  palatable  drink,  matzoon,  that 
has  considerable  dietetic  value  in  certain  stomach 
troubles. 

While  these  bacteria  are  undesirable  in  milk-  supplies, 
they  are  essential  to  the  production  of  butter  and  cheese. 
The  aroma  and  flavor  of  butter  is,  as  a  rule,  dependent 
upon  their  presence,  and  in  cheese  they  are  absolutely 
essential  in  various  stages  of  its  manufacture. 


Milch  Ztg.,  26:  212,  1897. 
2Kayser,  Cent.  f.  Bakt.,  II.  Abt.,    I:  436. 


Milk  Fermentations.  59 

58.  "  Gassy  "  milks.  This  very  common  and  undesir- 
able type  of  fermentation  is  caused  by  the  presence  of  a 
large  number  of  different  bacteria,  the  majority  of  which 
belong  to  the  lactic  acid  group.  The  amount  of  acid 
formed  is  always  considerably  less  than  that  which  is 
present  in  the  typical  lactic  formation.  Besides  this, 
there  is  also  produced  as  a  result  of  the  decomposition  of 
the  milk-sugar,  various  gases  such  as  CO2,  H,  and  meth- 
ane. Accompanying  these  are  also  to  be  noted  various 


FIG.  12.     A  cheese  made  from  "gassy"  milk.    A  severe  type  of  this 
fermentation. 

other  decomposition  substances  that  impart  to  milk  and  its 
products  an  undesirable  flavor  and  odor.  These  fermenta- 
tions are  particularly  undesirable  in  cheese-making,  as 
they  are  the  cause  of  "floating"  and  "pin-holey7'  curds.1 
The  swelling  or  huffing  of  green  cheese  is  invariably  due 
to  this  type  of  ferment  action.  Organisms  producing 
this  abnormal  change  are  particularly  numerous  in  ma- 
nure particles2,  the  colon  bacillus  (B.  coli  communisjy 

1  Freudenreich,    Landw.  Jahr.  d.  Schweiz,  p.  17,  1890. 
Russell,  12th  Kept.  Wis,  Expt.  Stat.,  p.  139,  1895. 

2  Bolley,  N.  D.  Expt.  Stat.,  Bull.  21. 


60  Dairy  Bacteriology. 

the  common  fecal  inhabitant  of  the  intestinal  canal  being 
able  to  produce  large  quantities  of  gas..    In  some  cases 
gas- generating  bacteria  belong  to  the  casein- dissolving 
group,  but  for  the  most  part  they  attack  the  milk-sugar. 
59.  Slimy  or  ropy  milk.     The  viscosity  of  milk  is 
often  markedly  increased  as  a  result  of  bacterial  fermenta- 
tions.    This  condition  varies  much  in 
intensity,   in    some    cases    the    milk 
merely  becoming  viscous,  when  it  is 
known  as  sticky  or  slimy;    then  again 
the  viscosity  may  be  so  increased,  that 
particles  of  milk,  when  touched,  will 
cohere  and  string  out  in  long  threads, 
in  which    condition    it   is   known    as 
ropy,  thready,  or  stringy.     This  con- 
dition in  milk  generally  occurs  during 
warm  weather.     Its  presence  prevents 
perfect  creaming,  as  the  globules   of 
butter-fat  are  unable  to  rise,  owing  to 
the  viscous  nature  of  the  fluid.     Nat- 
urally it  also   has  an  injurious  effect 
upon  the  quality  of  the  cream,  owing 
to  the  by-products  that  are  formed. 
While  a  number  of  different  species  of 
bacteria  have  been  more  or  less  thor- 
"  oughly  studied  which  possess  the  prop- 
erty of  rendering   milk   slimy,   these 
various  species  are  not  of  equal  im- 
FIG.  is.  siimy  milk.  pOrtance  in  affecting  milk  under  nat- 

This  milk  would  "string    *  to  . 

out"  several  feet  in  ural  conditions.    The  manner  in  which 
length>  the  slime-forming  organisms  are  first 

introduced  into  milk  is  of  considerable  importance,  as  a 
knowledge  of  this  often  enables  restrictive  measures  to  be 
applied  directly.    Marshall1  reports  an  outbreak  in  Michi- 
1  Marshall,  Mich.  Expt.  Stat.,  Bull.  140. 


Milk  Fermentations.  61 

gan  which  he  traced  to  an  external  infection  from  the 
udder.  In  another  case,  he  found  the  slimy  germ  in  the 
dust  on  the  floor  of  a  barn.  Guillebeau1  has  reported  a 
number  of  cases  in  which  slime-forming  organisms  were 
traced  to  a  diseased  condition  of  the  udder. 

In  Switzerland,  the  chief  cause  of  ropy  milk  seems  to 
be  Mic.  Freudenreichii,  a  large,  immotile,  non-liquefying 
coccus  that  grows  well  at  ordinary  temperatures.  This 
organism  is  readily  killed  by  heat,  two  minutes  at  212°  F. 
being  sufficient,  but  in  a  dried  condition  it  has  great 
resisting  powers. 

The  .slimy  substance  formed  in  milk  comes  from  vari- 
ous ingredients  of  the  milk,  and  the  chemical  character 
of  the  slime  produced  also  varies  with  different  germs.  In 
some  cases  the  slimy  material  is  merely  the  swollen  outer 
cell  membrane  of  the  bacteria  themselves;  in  others  it  is 
due  to  the  decomposition  of  the  proteids,  but  in  general, 
the  chief  decomposition  product  appears  to  come  from  a 
viscous  fermentation  of  the  milk-sugar. 

Normally  this  class  is  repressed  by  the  development  of 
the  lactic  acid  bacteria;  in  fact,  putrefactive  processes  sel- 
dom occur  where  the  lactic  organisms  are  in  the  ascend- 
ency. In  sterilized  or  pasteurized  milk  this  competition 
is  removed  as  the  lactic  forms  are  unable  to  withstand 
this  treatment,  while  this  enzyme-forming  group  sur- 
vive by  virtue  of  the  spores  which  they  possess.  They 
gain  access  to  the  milk  not  infrequently  from  manure 
particles2  that  are  derived  from  the  coat  of  the  animal. 

60.  Favorable  slimy  fermentations.  While  in  this 
country  slimy  milks  are  considered  as  undesirable,  yet  in 
certain  parts  of  Europe  changes  of  this  character  are  put 
to  good  use.  In  Holland  it  has  long  been  the  custom  to 

1  Guillebeau,  Landw.  Jahr.  d.  Schweiz,  p.  27,   1890. 
*  Backhaus,  abs.  in  Expt.  Stat.  Rec.,  10:  89. 


€2  Dairy  Bacteriology. 

add  a  starter  known  as  "lange  wei"  (long  or  stringy 
whey) ,  in  the  manufacture  of  Edam  cheese  to  control  the 
gassy  fermentations.  Weigmann  has  isolated  from  this 
material  a  slime-producing  organism  known  as  Strepto- 
coccus Hollandicus,  which  renders  milk  stringy  and  acid 
in  a  few  hours  at  ordinary  temperatures. 

The  clotted  or  thickened  milk  known  as  ' '  taettemjolk, ' ' 
that  is  a  favorite  beverage  in  Norway,  and  the  film j 61k 
(ropy  milk)  of  Finland,  is  produced  by  adding  leaves  of 
common  butterwort  (Pinguicula  vulgaris)  to  the  milk. 
Weigmann1  has  isolated  a  bacterial  form  from  the  leaves 
of  this  plant  that  is  similar  to  St.  Hollandicus  that  is  able 
to  cause  a  slimy  change  in  milk,  so  it  is  probable  the 
change  is  due  to  the  germs  on  the  surface  rather  than  to 
the  plant  itself. 

61.  Alcoholic  fermentations.  While  fluids  contain- 
ing ordinary  sugar  or  glucose  are  very  apt  to  undergo 
alcoholic  fermentation  if  exposed  to  the  air,  milk-sugar 
does  not  decompose  readily  in  this  way.  The  alcohol- 
producing  ferments  are  mainly  yeasts  which  class  of  or- 
ganisms do  not  thrive  readily  in  milk,  although  Duclaux2 
reports  a  serious  outbreak  in  a  dairy  due  to  an  organism 
of  this  class. 

Among  some  of  the  Oriental  tribes,  alcoholic  bever- 
ages are  used  that  are  make  from  milk.  Kumiss  (spelt 
also  koumiss,  kumys),  is  made  by  a  fermentation  of 
mare's  milk  induced  by  the  addition  of  old  kumiss. 

A  similar  preparation  intended  for  invalids  is  now 
made  in  this  country  from  cow's  milk  by  the  addition  of 
a  small  amount  of  cane-sugar  and  the  subsequent  intro- 
duction of  yeast. 


1  Weigmann,  Milch  Ztg.,  Beilage,  No.  48,  1889. 
3  Duclaux,  Principles  de  Laiterie,  p.  60. 


Milk  Fermentations.  63 

Another  alcoholic  beverage  that  is  in  common  use 
among  the  people  of  Caucasus  is  kephir  (also  kefyr,  kefir) . 
This  is  a  sour,  effervescent  alcoholic  fluid  prepared  from 
the  milk  of  goats,  cows,  or  sheep.  The  direct  cause  of 
the  fermentation  is  the  so-called  kefir  grain,  a  yellowish 
mass  about  as  large  as  a  walnut  that  is  added  to  the  milk. 
These  grains  are  left  in  the  milk  for  about  a  day;  the 
milk  is  then  poured  off,  and  the  grain  dried  and  pre- 
served for  future  use.  This  milk  after  being  mixed  with 
fresh  milk  is  kept  in  leather  flasks  and  soon  a  mixed  fer- 
mentation sets  in. 

This  alcoholic  change  has  been  studied  considerably 
from  a  biological  point  of  view1,  but  even  yet  is  not 
thoroughly  understood .  It  is  evidently  a  mixed  fermenta- 
tion, and  one  in  which  no  single  organism  can  produce 
all  the  essential  ingredients.  The  sugar  of  milk  is  par- 
tially converted  into  alcohol,  but  at  the  same  time  the 
casein  is  coagulated  and  digested  to  a  certain  extent,  so 
that  the  process  is  quite  complicated.  Some  of  the  or- 
ganisms isolated  have  been  found  to  be  able  to  produce 
a  change  allied  to  that  seen  under  natural  conditions.  A 
yeast  form  is  probably  the  main  cause  of  the  alcoholic 
fermentation,  while  bacteria  change  the  other  constitu- 
ents of  the  milk. 

62.  Butyric  acid  fermentations.  The  fermentation 
characterized  by  the  production  of  butyric  acid  is  also  a 
class  fermentation  that  is  caused  by  the  action  of  a  num- 
ber of  different  aerobic  and  anaerobic  bacterial  species. 
This  decomposition  process  is  common  in  boiled  milk  and 
as  a  secondary  fermentation  in  sour  milk.2  For  a  long 
time  it  was  thought  that  the  butyric  acid  change  in  sour 
milk  was  a  continuation  of  the  lactic  fermentation,  but 

1  Freudenreich,  Landw.  Jahr.  d.  Schweiz,  10:  1,  1896. 

2  Hofmann,  Ann.  de  Sci.  Nat.,  11:  5,  1869. 


64  Dairy  Bacteriology. 

it  is  now  believed  that  these  organisms  find  more  favor- 
able growth  not  so  much  on  account  of  the  lactic  acid 
formed  as  in  the  absence  of  dissolved  oxygen  in  the  milk 
which  is  consumed  by  the  sour  milk  organisms. 

Most  of  the  butyric  class  of  bacteria  are  spore-bearing, 
and  hence,  they  are  frequently  found  in  boiled  or  steril- 
ized milk .  The  by-products  formed  in  this  series  of  changes 
are  quite  numerous.  In  most  cases,  butyric  acid  is 
prominent,  but  in  addition  to  this,  other  organic  acids  as 
lactic,  succinic,  and  acetic  are  produced,  likewise  different 
alcohols.  Concerning  the  chemical  origin  of  butyric 
acid  there  is  yet  some  doubt.  Duclaux1  affirms  that  the 
fat,  sugar,  and  casein  are  all  decomposed  by  various  forms. 
In  some  cases,  the  reaction  of  the  milk  is  alkaline,  with 
other  species  it  may  be  neutral  or  acid.  This  type  of 
fermentation  has  not  yet  received  the  study  it  deserves. 

In  milk  these  organisms  are  not  of  great  importance, 
as  this  fermentation  does  not  readily  gain  the  ascendency 
over  the  lactic  bacteria.  It  has  been  supposed  until  re- 
cently, that  the  rancidity  of  butter  was  attributable  to 
the  action  of  these  ferments,  but  the  general  belief  now 
is  that  this  is  a  purely  chemical  change  that  may  be  in- 
fluenced but  not  caused  by  these  organisms.2 

63.  Bitter  milk.  A  bitter  taste  may  be  imparted  to 
milk  in  a  variety  of  ways.  In  some  cases  it  is  due  to 
improper  feeding  caused  by  eating  herbs  such  as  lupines, 
wormwood,  or  chicory.  Then  again,  at  certain  periods 
of  lactation,  a  bitter  salty  taste  is  occasionally  noted  in 
the  milk  that  is  peculiar  to  individual  animals. 

A  considerable  number  of  cases  of  bitter  milk  have, 
however,  been  traced  to  bacterial  origin.  For  a  number 
of  years  the  bitter  fermentation  of  milk  was  thought  to 

1  Duclaux,  Principes  de  Laiterie,  p.  67. 

2  Duclaux,  Compt.  rendu,  102:  1077. 


Milk  Fermentations.  65 

be  associated  with  the  butyric  fermentation,  but  Weig- 
mann1  showed  that  the  two  conditions  were  not  depend- 
ent upon  each  other.  He  found  that  the  organism  which 
produced  the  bitter  taste  acted  upon  the  casein. 

Conn2  found  a  coccus  form  in  bitter  cream  that  was 
able  to  impart  a  bitter  flavor  to  milk.  The  writer  sep- 
arated a  lactic  acid  species  from  milk  that  also  possessed 
a  similar  property.  Sometimes  a  bitter  condition  does 
not  develop  in  the  milk,  but  may  in  the  milk  products 
later.  Freudenreich3  separated  a  micrococcus  from  cheese 
that  was  found  to  be  the  cause  of  bitterness. 

Cream  ripened  at  low  temperatures  not  infrequently 
develops  a  bitter  flavor,  showing  that  the  optimum  tem- 
perature for  this  type  of  fermentation  is  below  the  typical 
lactic  acid  change. 

It  has  long  been  a  question  how  to  account  chemically 
for  the  bitter  taste  in  milk.  Various  ideas  have  been 
advanced,  but  Freudenreich  has  demonstrated  in  one  case 
that  a  bitter  substance  is  formed  in  the  milk  that  can  be 
isolated  by  adding  alcohol. 

Milk  that  has  been  cooked  is  likely  to  develop  a  bitter 
condition.  The  explanation  of  this  is  that  the  bacteria 
producing  the  bitter  substances  usually  possess  endospores 
and  that  while  the  boiling  or  sterilizing  of  milk  easily 
kills  the  lactic  acid  germs,  these  forms  on  account  of 
their  greater  resisting  powers  are  not  destroyed  by  the 
heat. 

64.  "  Sweet  curdling1"  and  digesting  fermenta- 
tions. Not  infrequently  milk  instead  of  undergoing 
spontaneous  souring  curdles  in  a  weakly  acid  or  neutral 
condition,  in  which  state  it  is  said  to  have  undergone 

1  Weigmann,  Milch  Ztg.,  1890,  p.  881. 

2  Conn,  Kept.  Storrs'  Expt.  Stat.,  p.  158.  1890. 

3  Freudenreich,  Fiihl.  Landw.  Ztg.,  43:  361. 

5-B. 


66  Dairy  Bacteriology. 

"  sweet  curdling. "  In  some  cases  the  curdled  casein  may 
remain  intact;  in  others  it  steadily  diminishes  in  volume, 
a  turbid  and  somewhat  colored  watery  fluid  separating 
from  it.  In  this  stage,  the  milk  is  said  to  have  "wheyed 
off.'1 

The  physical  appearance  of  milk  undergoing  these  fer- 
mentations is  materiall}*  different  from  that  where  normal 
souring  occurs.  The  curd  of  sour  milk  is  hard  and  breaks 
with  a  fractured  surface,  while  the  coagulum  in  these 
cases  is  soft  and  somewhat  slimy.  In  the  later  stages  of 
the  digestive  process,  the  milk  assumes  a  watery  appear- 
ance. 

These  fermentations  assume  two  phases:  1.  Curdling 
followed  by  a  subsequent  digestion  of  the  casein;  2. 
Digestion  or  peptonization  of  the  casein  without  any  ap- 
parent previous  coagulation. 

A  great  variety  of  bacteria  are  able  to  participate  in 
these  changes,  particularly  those  belonging  to  the  class 
represented  by  the  hay  and  potato  bacilli.  This  group  of 
bacteria  as  a  rule  are  able  to  liquefy  gelatin,  a  fermenta- 
tive change  of  a  similar  nature  to  the  digestion  of  the 
casein.1  In  fact  they  are  frequently  referred  to  as 
casein-  ferments.  The  characteristic  of  these  fermenta- 
tions is  the  production  of  certain  unorganized  ferments 
or  enzymes  that  have  the  power  of  acting  on  the  proteid 
molecule  independent  of  vital  activity.  The  two  ferments 
that  are  best  known  are  the  rennet  or  curdling  enzyme, 
and  the  tryptic  or  digesting  enzyme.  As  a  rule  any  or- 
ganism that  possesses  the  digestive  power,  first -causes  a 
coagulation  of  the  casein  in  a  manner  comparable  to  ren- 
net. Conn2  has  separated  this  enzyme  in  a  relatively 
pure  condition,  and  Fermi3  has  isolated  the  digestive 

1  Sterling,  Cent.  f.  Bakt.,  II.  Abt.,1:  473. 

2  Conn,  5th  Rep't.  Storrs'  Expt.  Stat.,  196,  1892. 

3  Fermi,  Arch.  f.  Hyg.,  14:   1,  1892. 


Milk  Fermentations.  67 

principle  from  a  number  of  different  species.  Duclaux1 
has  given  to  this  digesting  enzyme  the  name  casease  or 
cheese  ferment.  These  isolated  ferments  when  added  to 
fresh  milk  possess  the  power  of  causing  the  characteristic 
curdling  and  subsequent  digestion  quite  independent 
of  cell  development.  The  quantity  of  ferment  produced 
by  different  species  differs  materially  in  some  cases,  the 
amount  of  rennet  ferment  being  so  imperceptible  as 
to  be  obscured  in  the  reaction  by  the  digestive  process. 

In  these  fermentations,  the  chemical  transformations 
are  profound,  the  complex  proteid  molecule  being  broken 
down  into  albumoses,  peptones,  amido- acids  (tyrosin  and 
leucin),  and  ammonia  as  well  as  fatty  acids. 

Not  infrequently  these  fermentations  gain  the  ascend- 
ency over  the  normal  souring  change,  but  under  ordinary 
conditions  they  are  repressed,  as  they  are  unable  to  tolerate 
the  lactic  acid  group.  They  are,  however,  present  in  all 
milks  to  a  greater  or  less  extent,  as  can  be  seen  from  the 
fact  that  boiled,  sterilized,  or  -pasteurized  milks  invari- 
ably undergo  this  type  of  fermentation  on  account  of  the 
resistance  of  the  spore-bearing  species  that  remain  in  the 
milk  after  the  lactic  forms  are  killed. 

They  are  present  in  milks  to  a  larger  extent  in  summer 
than  winter,  on  which  account  it  is  much  more  difficult 
to  sterilize  milk  thoroughly  during  this  season.  Germs  of 
this  class  are  not  only  undesirable  if  they  gain  the  as- 
cendency in  milk,  but  where  milk  is  made  up  into  cheese, 
considerable  loss  occurs  from  the  digestion  of  the  casein, 
the  peptonized  portions  being  lost  in  the  whey. 

65.  Soapy  milk.  Weigmann  and  Zirn2  isolated  from 
a  milk  having  a  soapy  flavor,  a  specific  germ,  B.  lactis 
saponacei,  capable  of  imparting  a  taste  of  this  sort.  Milk 

1  Duclaux,  Le  Lait.  p.  121. 

2  Weigmann  and  Zirn,  Milch  Zeit.,  22:  569. 


68  Dairy  Bacteriology. 

affected  in  this  way  foams  readily,  and  is  abnormally 
slow  in  souring.  They  traced  the  cause  of  the  trouble  in 
one  case  to  the  infection  of  the  milk  from  the  straw  that 
was  used  for  bedding;  in  another,  it  was  present  in  the 
hay.  Marshall1  in  this  country  has  also  isolated  an  or- 
ganism of  this  sort  that  acts  upon  casein  and  albumen. 

66..  Bloody  or  red  milk.  This  condition  often  arises 
from  the  actual  presence  of  blood  in  the  milk  due  to 
some  wound  in  the  udder.  The  ingestion  of  certain  plants, 
as  sedges  and  scouring  rushes,  is  said  to  cause  a  bloody 
condition  in  milk;  madders  impart  a  reddish  tinge  on 
account  of  a  coloring  matter  absorbed.  These  instances 
can  always  be  separated  from  bacterial  troubles,  because 
if  due  to  these  sources  the  color  will  be  noted  at  the  time 
of  milking. 

There  are  several  chromogenic  or  pigment-bearing  bac- 
teria that  have  been  isolated  from  milk,  that  have  the 
power  of  turning  milk  red,  although  this  change  is  so 
slow  that  it  does  not  amount  to  much  in  dairy  practice. 
The  most  widely  known  form  able  to  bring  about  this 
change  is  Bacillus  prodigiosus,  the  so-called  "bleeding 
bread ' '  bacillus.  It  is  interesting  from  a  historical  stand- 
point, inasmuch,  as  it  has  been  found  to  be  the  cause  of 
the  bloody  bread  that  has  been  the  source  of  much  super- 
stitious fear.  Its  growth  in  milk  is  marked  by  the  pro- 
duction of  a  coloring  matter  that  is  diffused  throughout 
the  milk,  especially  near  the  upper  surface.  Free  con- 
tact with  oxygen  is  required  to  produce  the  characteristic 
pigment.  Bacillus  lactis  erythrogenes  (bacillus  of  red 
milk),  is  another  form  that  grows  easily  in  milk,  produ- 
cing a  red  color,  but  it  has  the  curious  property  of  being 
able  to  form  the  color  only  in  the  dark,  and  in  milk 

1  Marshall,  Mich.  Expt.  Stat.,  Bull.  146,  p.  16. 


Milk  Fermentations.  69 

that  is  not  strongly  acid  in  its  reaction.  When  grown 
in  the  light,  this  germ  forms  a  yellow  pigment.  In 
milk,  the  casein  is  slowly  precipitated  and  gradually  dis- 
solved. It  thrives  at  a  high  temperature,  from  80°-95°  F . 

Other  cases  of  red  milk  have  been  reported  that  have 
been  traced  to  other  germs.1  Menge2  found  a  red  sar- 
cina  that  was  the  cause  of  trouble  in  a  milk-supply. 
Here  in  this  country,  it  is  not  at  all  uncommon  to  find  on 
old  milk,  red  patches  that'  are  caused  by  the  growth  of  a 
yeast-like  germ,  Saccharomyces  glutinis,  that  is  often 
found  in  the  air. 

67.  Blue  milk.  Blue  milk  is  historically  a  better 
known  disease  than  almost  any  other  trouble  in  milk. 
As  long  ago  as  1838,  Steinhoff  showed  that  the  trouble 
was  communicable  from  one  lot  of  milk  to  another.  It 
manifests  itself  in  the  course  of  one  to  three  days  by  the 
appearance  of  isolated  flecks  of  bluish  or  grayish  color 
on  the  surface  of  the  milk.  In  fresh  milk  that  is  only 
slightly  acid,  the  gray  tints  prevail,  but  as  the  amount 
of  lactic  acid  increases  in  the  milk,  the  blue  coloration 
becomes  more  marked.  So  far  only  one  form  (Bacillus 
cyanogenus)  is  known  that  is  able  to  produce  this  change. 
It  does  not  materially  affect  the  milk,  but  the  butter 
made  from  infected  cream  has  very  poor  keeping  quali- 
ties. In  Mecklenburg  an  outbreak  of  this  disease  once 
persisted  for  a  period  of  several  years3.  Heim4  found 
that  this  bacillus  was  especially  resistant  toward  drying, 
or  the  influence  of  chemical  agents  like  soda  and  potash, 
but  a  temperature  of  176°  F.  for  a  moment  sufficed  to 
kill  it. 

1  Keferstein,  Cent.  f.  Bakt.  I  Abt.,  21:  177. 

2  Menge,  Cent.  f.  Bakt.,  6:  596. 

3  Fleischmann,  Book  of  the  Dairy,  p.  51. 

4  Heim,  Arb.  a.  d.  Kais.  Ges.  Amte,   5:  518. 


70  Dairy  Bacteriology. 

68.  Other  kinds  of  colored  milk.      Two  or  three 
chromogenic  forms  producing  still  other  colors  have  been 
found   in   milk.     Schroeter   discovered   in  a  sample  of 
cooked  milk,  a  peculiar  form  (Bacillus  synxanthus)  that 
produced  a  citron  yellow  appearance  and  which  precipi- 
tated and  finally  dissolved  the  casein.     Adametz,  Conn, 
and  List  have  described  other  species  that  confer  tints  of 
yellow  on  milk.     Some  of  these  are  bright  lemon,  others 
orange,  and  some  amber  in  color. 

Still  other  color- producing  bacteria,  such  as  those 
that  produce  violet  or  green  changes  in  the  milk  have 
been  observed.  In  fact,  almost  any  of  the  chromogenic 
bacteria  are  able  to  produce  their  color  changes  in  milk 
as  it  is  such  an  excellent  food  medium.  Under  ordinary 
conditions,  these  do  not  gain  access  to  milk  in  sufficient 
numbers  so  that  they  modify  the  appearance  of  it  except 
in  occasional  instances. 

69.  Treatment  of  milk  fermentations.    Attention 
has  already  been  drawn  to  the  distinction  that  should  be 
made  between  taints  due  to  fermentative  action  caused  by 
the  absorption  of  some  pre-existing  odor.     In  treating 
any  abnormal  fermentation,  the  attempt  should  first  be 
made  to  locate  the  cause  of  the  trouble.     In  most  in- 
stances where  the  difficulty  is  due  to  bacteria,  it  is  caused  by 
foreign  matter  gaining  access  to  the  milk.     Scrupulous 
cleanliness  will  therefore  in  most   cases  eliminate  the 
trouble  and  the  suggestions  made  in  Chapter  IV  as  to 
the  methods  of  preventing  bacterial  infection  will  be  help- 
ful in  this  connection.     So  efficacious  is  this  course  that 
cleanliness  in  every  detail  in  dairy  pursuits  is  almost  a 
panacea  for  troubles  and  taints  of  all  sorts  that  occur  in 
milk. 

If  the  taint  is  recognized  in  the  mixed  milk  of  the  herd, 
it  is  necessary  to  ascertain,  first,  whether  it  is  a  general 


Milk  Fermentations.  71 

trouble,  or  whether  it  is  restricted  to  one  or  more  ani- 
mals. For  this  purpose  the  fermentation  or  curd  test  in 
some  form  or  other  is  invaluable.  Often  the  whole  milk- 
ing may  be  infected  from  the  milk  of  a  single  animal,  as 
is  frequently  the  case  in  udder  inflammation. 

To  prove  whether  the  trouble  is  a  general  or  incidental 
one,  can  easily  be  done  by  separating  the  milk  of  the  dif- 
ferent cows,  or  if  this  is  not  feasible  by  massing  that  of 
a  few  together,  and  so  gradually  narrowing  down  the 
number. 

Where  the  trouble  is  a  general  one,  and  is  not  due  to 
the  spreading  of  the  infection  from  a  local  source,  the 
fault  is  usually  to  be  traced  to  some  error  in  handling 
the  whole  mass  of  milk.  Imperfectly  cleaned  cans  that 
are  used  in  setting  the  milk  often  contaminate  the  entire 
lot;  then,  too,  noxious  germs  derived  from  the  coat  of 
the  animal  often  gain  access  to  the  milk.  The  herd,  es- 
pecially in  the  late  summer,  when  the  upland  pastures 
are  dry  and  the  grass  is  short,  are  generally  pastured 
on  marsh  or  lowland  fields.  The  stock  seek  the  low  places 
— frequently  slime-covered  mud-holes,  and  in  passing 
through  these,  their  coats  are  fouled  with  the  scum  and 
slime  that  is  filled  with  putrefactive  forms  of  bacteria. 

Sometimes  the  source  of  the  filth  may  be  in  the  barn 
itself.  Dirty  stalls  filled  with  moist  and  decaying  mat- 
ter may  be  the  means  by  which  the  milk  is  often  seeded, 
or  it  may  come  from  manure  particles  that  contain  putre- 
factive organisms  in  abundance.  Generally  where  the 
source  of  contamination  can  be  discovered,  it  will  be  an 
easy  matter  to  get  rid  of  the  obnoxious  fermentation  by 
using  physical  means  of  disinfection  such  as  steam  or  hot 
water.  In  some  cases  pasteurizing  the  milk  is  of  material 
help.  Chemical  disinfection  can  sometimes  be  employed 
advantageously,  but  the  application  of  these  agents  should 


72  Dairy  Bacteriology. 

be  confined  to  the  treatment  of  surroundings,  rather  than 
used  in  the  milk  itself. 

70.  Overcoming1  taints  by  use  of  starters.    An- 
other method  that  is  often  fruitful,  is  that  which  rests 
upon  the  inability  of  one  kind  of  bacteria  to  grow  in  the 
same  medium  in  competition  with  other  species. 

Some  of  the  undesirable  taints  in  factories  can  be  in 
large  part  controlled  by  the  introduction  of  starters  made 
from  certain  organisms  that  are  able  to  obtain  the  as- 
cendency over  the  taint-producing  germ.  Such  a  method 
is  commonly  followed  when  a  lactic  ferment,  either  a 
commercial  pure  culture,  or  a  home-made  starter,  is  added 
to  milk  to  overcome  the  effect  of  gas- generating  bacteria. 

A  similar  illustration  is  seen  in  the  case  of  the  "lange 
wei"  (slimy  whey),  that  is  used  in  the  manufacture  of 
Edam  cheese  to  control  the  character  of  the  fermentation 
of  the  milk. 

This  same  method  is  sometimes  applied  in  dealing  with 
certain  abnormal  fermentations  that  are  apt  to  occur  on 
the  farm.  It  is  particularly  useful  with  those  tainted 
milks  known  as  "sweet  curdling. n  The  ferment  organ- 
isms concerned  in  this  change  are  unable  to  develop 
in  the  presence  of  lactic  acid  bacteria,  so  the  addition  of 
a  clean  sour  milk  as  a  starter  restores  the  normal  condi- 
tions by  giving  the  ordinary  milk  bacteria  the  ascend- 
ency. 

71.  Chemical  disinfection.     In  only  exceptional  in- 
stances will  it  be  necessary  to  employ  chemical  disinfect- 
ants to  restore  the  normal  conditions.     Of  course  with 
such  diseases  as  tuberculosis,  very  stringent  measures  are 
required,  as  they  are  such  a  direct  menace  to  human  life, 
but  with  these  abnormal  or  taint- producing  fermenta- 
tions, care  and  cleanliness,  well  directed,  will  usually 
overcome  the  trouble. 


Milk  Fermentations.  73 

In  case  it  becomes  necessary  to  employ  chemical  sub- 
stances as  disinfecting  agents,  their  use  should  always  be 
preceded  by  a  thorough  cleansing  with  hot  water  so  that 
the  germicide  may  come  in  direct  contact  with  the  surface 
to  be  disinfected. 

It  must  be  borne  in  mind  that  many  chemicals  act  as 
deodorants,  i.  e.,  destroy  the  offensive  odor,  without  de- 
stroying the  cause  of  the  trouble. 

Sulfur  is  often  recommended  as  a  disinfecting  agent, 
but  its  use  should  be  carefully  controlled,  otherwise  the 
vapors  have  but  little  germicidal  power.  The  common 
practice  of  burning  a  small  quantity  in  a  room  or  any  closed 
space  for  a  few  moments,  has  little  or  no  effect  upon  germ 
life.  The  effect  of  sulfur  vapor  (802)  alone,  upon  germ 
life  is  relatively  slight,  but  if  this  gas  is  produced  in  the 
presence  of  moisture,  sulfurous  acid  (H2  S03)  is  formed, 
which  is  very  much  more  efficient.  To  use  this  agent 
effectively,  it  must  be  burned  in  large  quantities  in  a 
moist  atmosphere  (three  Ibs.  to  every  1,000  cubic  feet  of 
space)  for  at  least  twelve  hours.  After  this  operation, 
the  space  should  be  thoroughly  aired. 

Formalin,  a  watery  solution  of  a  gas  known  as  form- 
aldehyde is  a  new  disinfectant  that  recent  experience  has 
demonstrated  to  be  very  useful.  It  may  be  used  as  a 
gas  where  rooms  are  to  be  disinfected,  or  applied  as  a 
liquid  where  desired.  It  is  much  more  powerful  in  its 
action  than  sulfur,  and  it  has  a  great  advantage  over 
mercury  and  other  strong  disinfectants,  as  it  is  not  so 
poisonous  to  man  as  it  is  to  the  lower  forms  of  life. 

Bleaching  powder  or  chloride  of  lime  (Ca012)  is  often 
recommended  where  a  chemical  can  be  advantageously 
used.  This  substance  is  a  good  disinfectant  as  well  as  a 
deodorant,  and  if  applied  as  a  wash,  in  the  proportion  of 
four  to  six  ounces  of  the  powder  to  one  gallon  of  water, 


74  Dairy  Bacteriology. 

it  will  destroy  most  forms  of  life.  In  many  cases  it  is 
inapplicable  on  account  of  its  odor. 

Corrosive  sublimate  (HgCl2)  for  most  purposes  is  a 
good  disinfectant,  but  it  is  such  an  intense  poison  that  its 
use  is  dangerous  in  places  that  are  at  all  accessible  to  stock. 

For  the  disinfection  of  walls  in  stables  and  barns,  com- 
mon thin  whitewash  (CaOH)  if  made  from  freshly  burned 
quicklime  is  admirably  adapted.  It  possesses  strong 
germicidal  powers,  increases  the  amount  of  light  in  the 
barn,  is  a  good  absorbent  of  odors,  and  is  exceedingly 
cheap. 

Carbolic  acid,  creosote,  and  such  products,  while  excel- 
lent disinfectants,  cannot  well  be  used  in  factories  on  ac- 
count of  their  odor. 

For  gutters,  drains,  and  waste-pipes  in  factories,  vitriol 
salts  (sulfates  of  copper,  iron,  and  zinc)  are  often  used 
These  are  deodorants  as  well  as  mild  disinfectants. 


CHAPTER  VI. 
DISEASE-PRODUCING  BACTERIA  IN  MILK. 

73.  Milk  a  medium  for  pathogenic  bacteria.    Not 

only  is  milk  an  excellent  food  for  fermentative  bacteria, 
but  a  not  inconsiderable  number  of  disease  organisms 
also  find  in  it  a  suitable  substratum  for  development.  In 
so  far  as  these  occur  in  milk  under  natural  conditions, 
they  become  a  serious  menace  to  public  health.  Statis- 
tical evidence  shows  that  quite  a  large  number  of  epi- 
demics have  been  traced  to  a  contaminated  milk-supply, 
which  has  served  as  a  vehicle  for  the  transmission  of  dif- 
ferent disease  organisms.  The  following  table,  prepared 
by  Freeman,  represents  the  outbreaks  that  have  occurred 
since  1880. 

Number  of  disease  epidemics  traced  to  contaminated  milk- 
supply. 

Typhoid  fever 53     Foot  and  mouth  disease 2 

Scarlet  fever 26     Throat  trouble 3 

Diphtheria 11     Cholera    1 

While  such  evidence  can  only  be  approximately  correct, 
still  the  data  at  hand  is  overwhelming  as  to  the  relation 
of  disease  bacteria  to  milk. 

These  organisms  may  be  considered  under  the  follow- 
ing heads : 

1.  Pathogenic   bacteria    causing  diseases  common  to 
man  and  beast,  which  may  be  transmitted  directly  to 
man  through  the  medium  of  the  milk,  as  in  tuberculosis. 

2.  Pathogenic  bacteria  that  can  thrive  in  milk  under 
saprophytic  conditions,  and  which  gain  access  to  it  sub- 
sequent to  its  withdrawal,  as  in  typhoid  fever. 

[751 


76  Dairy  Bacteriology. 

3.  Saprophytic  bacteria  that  can  form  toxic  or  poison- 
ous substances  in  the  milk  itself  or  in  the  intestine  after 
it  is  ingested,  as  in  certain  types  of  cholera  infantum. 

A.    DISEASE  BACTERIA  DERIVED  DIRECTLY  FROM  AF- 
FECTED ANIMAL. 

74.  Tuberculosis.  Of  those  diseases  that  are  com- 
municable from  the  animal  to  man  by  means  of  the  milk, 
tuberculosis  is  by  far  the  most  common.  This  term  is 
now  used  to  indicate  a  number  of  maladies  that  have 
heretofore  been  claimed  as  separate  diseases  and  which 
affect  warm-blooded  animals.  It  is  now  known  that  these 
different  manifestations  are  all  caused  by  the  growth  of 
the  tubercle  bacillus,  which  was  discovered  by  Koch  in 
1882.  In  this  connection,  reference  can  only  be  made  to 
the  bovine  type  of  the  disease,  and  the  relation  that  this 
bears  in  milk  and  dairy  products  to  the  human  race. 
The  disease  is  caused  by  the  same  germ  whether  it  is 
present  in  the  human  being  or  the  lower  animals,1  and 
the  danger  of  infection  exists  in  the  transmission  of  the 
virus  from  one  to  the  other. 

The  organism  causing  this  disease  is  remarkable  for 
the  narrow  temperature  limits  within  which  growth  will 
take  place,  the  minimum  being,  according  to  Koch,  86° 
F.,  while  the  maximum  is  104°  F. 

This  fact  is  of  importance  as  it  indicates  that  the  tuber- 
cle bacillus  is  unable  to  develop,  under  normal  tempera- 
ture conditions,  in  milk  after  it  is  drawn.  The  organism 
withstands  drying  readily;  in  fact,  by  virtue  of  this  prop- 
erty, it  is  most  widely  disseminated,  as  the  tubercular 
material  is  discharged  from  the  diseased  animal  and  is 
distributed  in  a  dried  condition  in  the  dust. 


1  Th.  Smith  has  recently  determined  that  there  are  certain  varietal 
differences  between  these  two  types  of  tuberculosis. 


Disease  Bacteria  in  Milk.  77 

Putrefaction  and  decomposition  even,  will  not  quickly 
destroy  its  vitality.  Direct  sunlight  is,  however,  an  effi- 
cient disinfecting  agent,  and,  even  in  diffused  light  the 
bacilli  are  destroyed  in  a  few  days. 

75.  Prevalence  among*  cattle.      Tuberculosis   not 
infrequently  affects  many  species  of  warm-blooded  ani- 
mals, but  it  is  particularly  prevalent  among  cattle  in  cer- 
tain regions  of  the  world. 

The  recent  introduction  of  the  tuberculin  of  Koch  as 
an  aid  in  the  diagnosis  of  the  disease  in  cattle  has  shown 
the  trouble  to  be  more  widely  spread  than  was  at  first 
believed,  but  sufficient  data  have  not  yet  been  collated 
to  enable  an  accurate  estimate  to  be  made. 

In  Denmark  the  percentage  of  affected  animals  has 
been  shown  by  Bang  to  be  very  high,  ranging  from 
30-40% .  Concerning  the  distribution  of  the  disease  in  this 
country,  data  is  still  very  meager.  In  15,000  head  tested 
under  the  auspices  of  the  U.  S.  Dept.  of  Agriculture, 
19%  showed  a  reaction.  Unquestionably  it  is  more 
widely  disseminated,  and  a  larger  percentage  of  stock  are 
affected  in  the  older  dairy  regions  of  the  east  than  in  the 
west.  The  amount  of  disease  among  the  range  stock  of 
the  western  plains  is  relatively  small. 

76.  Tuberculin  test.     The  tuberculin  test  is  made  by 
injecting  into  the  animal  a  small  quantity  of  a  liquid 
known  as  tuberculin,  which  is  a  sterile  glycerine  extract 
of  the  growth  products  of  the  tubercle  bacillus.     The  re- 
sults obtained  by  the  use  of  this  test  show  that  it  is  far 
superior  to  the  physical  methods  of  examination,  it  being 
possible  to  detect  the  disease  in  its  earliest  stages.  When 
tuberculin  is  inoculated  into  an  animal  it  causes  a  febrile 
reaction  in  those  cases  affected  with  this  disease;   the 
rise  in  temperature  usually  exceeding  the  average  normal 


78 


Dairy  Bacteriology. 


temperature  three  or  four  degrees;  in  healthy  animals 
only  a  slight  rise  occurs.  A  positive  reaction  should  be 
at  least  from  2.2-2.5°  F.  above  average  normal,  and 
should  be  maintained  with  some  fluctuations  for  several 
hours.  Animals  in  the  later  stages  of  the  disease  some- 
times do  not  respond,  but  when  the  test  is  properly  ap- 
plied, it  is  on  the  whole  a  most  satisfactory  aid  in  diag- 
nosis. 

77.  Relation  of  disease  organism  to  milk-supply. 
While  tuberculosis  is  widely  distributed  among  cattle,  it 


FIG.  14.    Side  view  of  tuberculous  udder,  showing  extent  of  swelling  in  single 
quarter. 

must  not  be  supposed  that  the  milk  of  all  reacting  ani- 
mals is  infected  with  the  specific  organism.  In  a  small 
percentage  of  cases,  the  udder  and  related  tissues  are  af- 
fected, in  which  case  the  milk  generally  possesses  in- 


Disease  Bacteria  in  Milk.  79 

factious  qualities.  In  the  early  stages  of  the  disease,  the 
milk  appears  perfectly  normal;  as  the  disease  progresses 
the  milk  assumes  a  watery  appearance  and  changes  in 
color.  A  diseased  condition  of  the  udder  can  usually  be 
diagnosed  where  a  hard,  painless  swelling  of  the  udder 
occurs  that  is  confined  to  one  quarter.  Udder  inflamma- 
tions other  than  tubercular  are  generally  accompanied 
with  pain.  Milk  from  animals  having  udder  tuberculo- 
sis should  be  unconditionally  rejected  for  food  purposes. 

Woodhead1  found  in  fourteen  out  of  nineteen  cases 
that  milk  from  a  tuberculous  animal  or  the  sediment 
from  it  was  sufficiently  infectious  to  produce  the  disease 
in  guinea-pigs  inoculated  with  small  quantities  of  it. 

The  writer2  found  one  case  in  which  a  single  cc.  of 
milk  from  a  diseased  animal  sufficed  to  kill  a  rabbit  in- 
oculated with  it.  In  this  same  case,  the  bacilli  were  also 
demonstrated  microscopically  in  the  milk. 

Sometimes  the  udder  contains  tubercle  bacilli  and  still 
does  not  show  any  external  symptoms  of  the  disease. 
Bang3,  Ernst4,  and  others  have  demonstrated  that  in 
quite  a  percentage  of  animals  with  apparently  healthy 
udders,  the  milk  possesses  infectious  properties. 

Where  the  disease  is  localized  in  the  lungs,  the  danger 
from  the  milk  is  probably  but  slight,  but  it  is  often  im- 
possible to  determine  the  exact  condition  of  the  disease 
in  the  animal.  Even  where  the  disease  is  local  there  is 
a  danger  that  a  previous  chronic  condition  may  suddenly 
become  acute. 

With  adults  in  normal  health,  the  danger  from  an  in- 
fected milk-supply  is  undoubtedly  greatly  minimized,  as 

1  Woodhead,  Trans.  7th  Intern.  Hyg.  Cong.,  London,  1891. 
8  Russell,  llth  Kept.  Wis.  Expt.  Stat.,  p.  196,  1894. 
8  Bang,  Cong,  for  Tuberculosis,  1888,  p.  70. 
4  Ernst,  Hatch  Expt.  Stat.,  Bull.  8,  1890. 


80  Dairy  Bacteriology. 

the  healthy  digestive  tract  is  relatively  insusceptible,  but 
with  infants  and  invalids  the  case  is  far  different.  The 
difficulty  of  proving  an  infection  in  this  way  is  very 
great,  as  it  is  impossible  to  exclude  the  more  common  in- 
fection by  the  air;  yet  a  number  of  well  authenticated 
cases  have  been  traced  to  this  source.  The  presence  of 
the  disease  in  many  cases  in  the  digestive  organs  alone  is 
inexplicable  except  in  this  way.  It  is  impossible  to  de- 
termine what  percentage  of  human  tuberculosis  is  ac- 
quired in  this  way,  but  even  though  the  amount  be  small, 
it  devolves  upon  us  to  restrict  this  possibility  in  every 
conceivable  way.  Not  alone  should  it  be  considered  from 
the  human  standpoint,  but  from  the  point  of  view  of 
successful  animal  industry  must  every  measure  be  taken 
to  repress  and  extirpate  this  disease  from  our  herds. 

Tubercle  bacilli  in  other  milk  products.  If  milk  con- 
taining numerous  tubercle  bacilli  is  made  up  into  differ- 
ent products  the  specific  organism  still  is  able  to  exist  in 
such  for  a  considerable  length  of  time.  Heim1  found 
that  tubercle  bacilli  were  able  to  live  in  butter  for'  a 
month  and  in  cheese  for  a  fortnight,  but  the  probability 
of  infection  occurring  in  this  way  is  very  slight  as  the 
quantity  of  these  materials  consumed  at  any  one  time  is 
not  large. 

78.  Methods  of  treating-  milk.  The  possible  danger 
from  tuberculosis  being  spread  by  a  contaminated  milk- 
supply  may  be  greatly  diminished  or  entirely  eliminated 
in  the  following  ways: 

1.  Dilution.  To  produce  infection  it  requires  the 
simultaneous  introduction  of  a  number  of  organisms. 
Bollinger  and  Gebhardt2  showed  that  milk  from  tuber- 
culous animals  that  would  produce  the  disease  in  guinea- 

1  Heim,  Arb.  a.  d.  kais.  Gesundh.,  5:  294. 

2  Bollinger,  Tagebl.  d.  62  Versamm.   Deutsch.  Naturf.,  Sept.  1889. 


Disease  Bacteria  in  Milk.  81 

pigs  and  rabbits  was  innocuous  when  diluted  with  healthy 
milk  to  50-100  times  its  volume.  Therefore,  there  is  less 
danger  in  mixed  herd  milks,  than  in  that  of  a  single  cow, 
unless  it  is  positively  known  that  she  is  unaffected  with 
the  disease. 

2.  Destruction  by  heat.     If  milk  containing  tubercle 
bacilli  is  boiled,  the  disease  organism  is  destroyed.     The 
same  result  may  be  accomplished  by  pasteurizing  if  this 
process  is  correctly  carried  out.     A  temperature  of  155°- 
160°  F.  for  fifteen  to  twenty  minutes  is  sufficient  for  this 
purpose. 

3.  It  has  been  suggested  that  inasmuch  as  centrifuging 
seems  to  throw  out  many  of  the  tubercle  bacilli  in  the 
slime1  that  this  method  of  purification  could  be  used,  but 
Moore2  has  shown  that  no  reliance  can  be  placed  on  this 
method. 

79.  Foot  and  mouth  disease.  This  disease  is  dis- 
tinctively an  animal  malady,  but  it  is  also  transmissable 
to  man,  although  it  is  not  often  fatal.  Hertwig3  records 
some  experiments  made  on  himself.  After  drinking  milk 
from  an  animal  suffering  from  this  disease,  the  mucous 
membrane  of  the  mouth  became  swollen,  small  vesicles 
appearing  in  the  mouth.  Not  an  inconsiderable  number 
of  epidemics  have  been  reported  that  have  been  traced  to 
this  source.4  Calves  readily  acquire  the  disease  suckling 
affected  mothers.  The  law  in  Prussia  since  1894  requires 
that  the  milk  from  all  infected  herds  shall  be  heated  to 
194°  F.,  for  fifteen  minutes  before  it  is  taken  from  the 
dairy. 

1  Bang,  Milch  Zeitung,  1893,  p.  672. 
8  Moore,  Year  Book,  Dept.  of  Agriculture,  p.  432,  1895. 
3Ziemssen,  Cyclop,  of  Prac.  of  Med.,  3:  521. 
4  Freeman,  Med.  Record,  March  28,  1896. 
6— B. 


82  Dairy  Bacteriology. 

80.  Other  diseases.     There  are  a  number  of  other  bo  - 
vine  diseases  such  as  anthrax, 1  lockjaw, 2  and  hydrophobia 
in  which  it  has  been  asserted  that  the  virus  of  the  disease  is 
transmissible  to  man,  but  in  most  cases  of  these  diseases 
the  secretion  of  the  udder  soon  becomes  affected,  so  that 
no  danger  need  be  apprehended  unless  such  milks  are 
wilfully  sold  for  human  food.     The  only  safe  rule  is  to 
reject  milk  coming  from  animals  that  show  any  signs  of 
sickness,  even  though  the  disease  may  be  one  that  is  not 
common  to  man. 

B.     INFECTION  OF  MILK  BY  DISEASE  BACTERIA  AFTER 
IT  IS  DRAWN. 

81.  Typhoid  Fever.      This  disease   stands   next   to 
tuberculosis  in  importance  in  its  relation  to  milk.     The 
organism  producing  this  fever  does  not  develop  in  the 
animal  itself,  consequently,  no  danger  need  be  appre- 
hended from  milk,  if  it  is  properly  cared  for  after  it  comes 
from  the  cow.      The  typhoid  fever   bacillus,  however, 
finds  in  raw  milk  such  favorable  conditions  for  develop- 
ment, that  if  it  is  once  introduced,  it  is  often  able  to 
thrive  for  a  considerable  length  of  time.     The  disease 
usually  spreads  by  means  of  the  water-supply,  yet  a  large 
number  of  epidemics  have  been  traced  directly  to  milk  as 
the  original  and  only  source  of  infection.     A  striking 
case  is  the  Stamford,   Conn.,   epidemic  of  1895.      386 
cases  of  the  disease  developed  in  six  weeks,  and  of  this 
number,  over  97%  came  from  a  single  milk-supply.  The 
milk  was  infected  by  rinsing  out  the  cans  with  water 
from  a  shallow  contaminated  well. 

The  contamination  of  milk  by  this  disease  has  been 


1  Stohmann,  Milch  u.  Molk.  produkte,  p.  382. 

2  Marx,  Deutsche  Viertelsjahr.  f.  offentl.  Gesunds.  Pflege,  20:  444, 
1890. 


Disease  Bacteria  in  Milk.  83 

traced  to  numerous  causes,  among  which  the  following 
are  most  important: 

1.  Direct  transmission  to  the  milk  from  person  conva- 
lescing from  the  disease. 

2.  Indirect   transmission    by  milker    also   serving   as 
nurse  to  patient. 

3.  Indirect  transmission  through  polluted  water-sup- 
ply where  same  is  used  for  cleansing  milk  vessels. 

All  of  these  sources  of  infection  can  be  readily  gov- 
erned with  a  little  care.  If  typhoid  fever  is  present  in 
the  family  of  a  milk-handler,  especial  care  should  be 
taken  that  no  one  has  any  access  to  the  patient  that  has 
anything  to  do  with  the  milk.  In  case  of  disease  in  fam- 
ily, all  water  used  in  cleaning  cans  should  first  be  boiled 
or  else  secured  from  a  source  in  which  contamination  is 
impossible.  Wells  in  vicinity  of  dwellings  are  often 
infected  with  disease  germs  that  are  derived  from  the  ex- 
creta, and  if  such  water  is  used,  infection  of  milk- supply 
is  possible. 

In  some  cases,  the  disease  has  been  spread  by  infect- 
ing a  general  creamery  supply,  the  contaminated  skim- 
milk  distributing  the  virus  of  the  disease.  In  the  Wei- 
ply  epidemic  in  England  in  1893,  twenty-three  cases 
appeared  among  the  patrons  of  a  single  factory. 

The  typhoid  fever  organism  is  able  to  thrive  in  milk 
for  a  considerable  time  (20-35  days  according  to  Heim)  * , 
on  account  of  its  tolerance  toward  weak  acids.  Even  in 
butter  and  cheese  these  organisms  are  able  to  live,  but 
the  conditions  are  so  unfavorable  and  the  probability  of 
infection  so  slight  as  to  practically  eliminate  such  a 
source . 

82.  Cholera,  diphtheria,  scarlet  fever,  etc. 

1.   Cholera.     Milk  also  functions  as  a  medium  for  the 

1  Heim,  Arb.  a.  d.  k.  Ges.  Amte,  5:  303. 


84  Dairy  Bacteriology. 

transmission  of  cholera,  although  the  danger  from  this 
source  is  somewhat  minimized  on  account  of  the  inability 
of  this  germ  to  thrive  luxuriantly  in  acid  fluids.  Kitasato1 
found  that  the  cholera  germ  could  live  in  raw  milk  from 
one  to  four  days,  depending  upon  the  amount  of  acid  that 
is  present  in  the  m ilk .  In  boiled ,  or  sterilized  milk ,  cholera 
can  grow  more  freely  as  the  acid-producing  organisms 
are  destroyed  by  the  heat  used.  In  butter  it  has  been 
found  after  four  or  five  days,  but  the  acid  reaction  re- 
strains its  growth  and  soon  kills  it. 

Cholera  epidemics  in  India  have  more  than  once  been 
traced  directly  to  contaminated  milk.  Simpson2  records 
a  case  of  infection  where  ten  sailors  out  of  twenty- four 
partook  of  some  milk  that  had  been  purchased  from  a 
milkman,  in  whose  district  cholera  had  recently  broken 
out.  Of  these  ten,  four  died,  five  were  severely  sick, 
and  one  that  used  the  milk  sparingly  was  slightly  ill. 
Investigation  proved  that  the  milk  had  been  adulterated 
with  water  that  had  been  taken  taken  from  an  open  pool 
in  the  infected  district. 

2.  Diphtheria.  This  is  another  contagious  disease,  the 
specific  organism  of  which  finds  in  milk  favorable  condi- 
tions of  growth,  and  there  is  abundant  evidence  to  indicate 
that  milk  functions  as  a  transmitter  of  contagion  if  it 
once  becomes  contaminated  with  the  same.  Whether  the 
animal  can  actually  serve  directly  to  spread  the  disease 
is  yet  a  question.  Klein3  affirms  as  a  result  of  animal 
inoculations  that  diphtheria  will  develop  in  the  cow,  at- 
tacking among  other  organs  the  udder,  and  so  infecting 
the  milk,  but  both  Abbott4  and  Vladimirow5  failed  to 
confirm  these  observations. 

1  Kitasato,  Arb.  a.  d.  Kais.  Gesund.  Amte,  1:  470. 
8  Simpson,  London  Practitioner,  39:  144  (1887). 

3  Klein,  19th  Kept.  Soc.  Gov.  Bd.  (Great  Britain),  1889,  167. 

4  Abbott,  Vet.   Mag.,  1:  17. 

5  Vladimirow,  Arch.  sci.  biol.  Inst.  Med.  St.  Petersburg,  p.  84,  1892. 


Disease  Bacteria  in  Milk.  85 

3.  Scarlet  Fever.  The  exact  relation  of  scarlet  fever 
to  milk  is  still  harder  to  study,  inasmuch,  as  the  specific 
organism  of  this  disease  has  not  yet  been  isolated.  Nev- 
ertheless, the  evidence  in  a  constantly  increasing  number 
of  epidemics  is  so  strong  in  favor  of  the  view  that  dis- 
semination of  disease  occurs  by  means  of  polluted  milk, 
that  the  relation  may  be  considered  as  practically  estab- 
lished. Klein1  holds  that  a  certain  eruptive  udder  dis- 
ease of  cattle  is  the  same  as  scarlet  fever,  and  that  it  is 
communicable  to  man.  His  conclusions,  however,  are 
not  generally  accepted,  although  it  is  regarded  as  thor- 
oughly proven  that  the  disease  may  be  propagated  in  the 
milk  after  it  is  drawn. 

C.     POISON-FORMING  BACTERIA  IN  MILK. 

83.  Toxic  or  poisonous  milk.  Milk  not  infre- 
quently acquires  poisonous  properties  by  virtue  of  the 
development  of  various  putrefactive  bacteria  that  form 
poisonous  by-products  as  a  result  of  decomposition  pro- 
cesses. In  some  cases  the  toxic  products  are  formed  in 
the  milk  outside  of  the  body.  When  such  milk  is  in- 
gested, symptoms  of  poisoning  soon  appear.  In  other 
cases,  certain  bacteria  found  in  the  milk  develop  in  the 
intestine  producing  a  toxic  effect. 

Milk  contaminated  with  filth  frequently  contains  putre- 
factive bacteria  that  form  substances  that  cause  gastric 
and  intestinal  troubles,  especially  in  infants.  Vaughan 
asserts  that  the  larger  number  of  cases  of  summer  diar- 
rhoea are  due  to  putrefactive  organisms  in  the  milk. 
The  much  higher  mortality  of  bottle-fed,  compared  with 
breast-fed  infants  is  undoubtedly  attributable  to  the  ac- 
tion of  these  microbes  in  milk2 . 

1  Klein,  7th  Intern.  Hyg.  Cong.  (London),  p.  130,  1891. 
2Baginsky,  Hyg.  Rund.,  p.  176,  1895. 


86  Dairy  Bacteriology. 

Many  of  the  cases  of  poisoning  due  to  ice  cream,  cheese, 
and  milk  come  from  such  a  source.  Improperly  handled 
milk  may  be  made  up  into  these  various  products,  and 
in  this  condition,  the  poisonous  principle  remain  un- 
changed for  a  considerable  time.  Vaughan1  has  isolated 
a  poisonous  substance  from  these  materials,  calling  it 
tyrotoxicon  (cheese  poison).  Our  knowledge  concerning 
the  way  in  which  it  is  produced  is,  as  yet,  very  vague, 
but  from  his  most  recent  researches2,  it  is  quite  probable 
that  the  organisms  causing  these  troubles  are  putrefac- 
tive forms  that  gain  access  to  the  milk  where  it  is  im- 
properly kept. 

1  Vaughan,  9th  Intern.  Hyg.  Cong.,  (London),  p.  118,  1891. 

2  Vaughan  and  Perkins,  Arch.  f.  Hyg.,  27:  308. 


CHAPTER  VII. 
PRINCIPLES  OF  MILK  PRESERVATION. 

84.  Keeping*  quality.     Where  milk  is  consumed  in 
the  form  of  milk  or  cream,  it  is  desirable  to  retard  or  in- 
hibit the  fermentative  changes  as  much  as  possible.     In 
the  preceding  chapters,  the  direct  relation  which  exists 
between  the  keeping  quality  of  the  milk  and  the  germ 
life  of  the  same  has  been  indicated,  so  that  it  is  evident, 
in  order  to  perfectly  preserve  milk,  the  bacteria  must 
either  be  prevented  from  gaining   an   entrance,  or  de- 
stroyed  after   they   have   once   established   themselves. 
Bacteria  are  so  widely  distributed,  and  milk  is  such  a 
nutritious  medium  for  their  development,  that  to  entirely 
prevent  their  entrance  and  growth  is  not  feasible.     Scru- 
pulous care  in  the  dairy  both  before  and  after  the  milking, 
will  however,  do  much  to  reduce  the  germ   content  of 
milk  as  is  seen  in  the  ' '  sanitary  "  or  "  certified ' '  milk 
that  is  being  put  on  the  market  in  some  cities.     These 
milks  are  drawn  under  the  most  careful  conditions,  and 
then  held  during  distribution  in  such  a  way  as  to  retard 
the  course  of  the  fermentative  changes  in  a  marked  de- 
gree.    Milk  so  treated  will  remain  sweet  for  several  days, 
which  period  is  sufficiently  long  for  practical  purposes. 
Such  milk  cannot  well  be  produced  in  herds  that  are  not 
under  one  management  as  the  degree  of  care  necessary  to 
success  is  not  often  exercised  unless  under  the  direct  su- 
pervision of  a  competent  manager. 

85.  Preservation  of  milk.    In  considering  methods 
of   milk  preservation,  reference   will  only  be   made   to 

[87] 


88  Dairy  Bacteriology. 

those  that  are  applicable  to  milk  intended  for  food.  Sam- 
ples destined  for  analytical  use,  such  as  for  fat  tests,  etc., 
would  be  treated  in  a  different  manner.  The  various 
methods  that  have  been  suggested  may  be  classified  un- 
der two  heads,  those  attained  by  the  use  of  chemical,  or 
by  physical  agents.  In  some  methods  the  preservation 
is  accomplished  by  the  operation  of  both  physical  and 
chemical  factors. 

86.  Chemical  agents.  Under  the  head  of  antiseptics 
and  disinfectants,  the  action  of  different  chemicals  on 
bacterial  life  has  been  discussed1 .  Those  substances  that 
are  inimical  to  the  development  of  bacteria  are  usually 
too  strong  for  use  in  a  food  product  as  preservatives, 
because  the  protoplasm  of  bacteria  as  a  rule  is  more  re- 
sistant than  other  forms  of  living  matter. 

For  this  reason,  the  application  of  strong  disinfectants 
like  carbolic  acid,  mercury  salts,  strong  acids,  or  alka- 
lies is  excluded.  The  chemical  agents  that  are  used  in 
the  preservation  of  milk  fall  into  two  classes. 

1.  Those  that  unite  chemically  with  certain  products 
of  bacterial  growth  to  form  more  or  less  inert  substances 
in  the  milk. 

2.  Those  that  restrain  or  inhibit  the  development  of 
fermentative  organisms  in  the  milk. 

To  this  first  class  belong  those  alkaline  salts  such  as 
bicarbonate  of  soda,  etc.,  that  combine  with  the  acids 
that  are  formed  in  the  milk.  While  salts  of  this  sort 
neutralize  the  acidity  produced  by  the  sour  milk  bacteria, 
they  do  not  kill  these  organisms,  their  growth  again 
being  renewed  after  the  milk  is  neutralized. 

As  representatives  of  the  second  class,  salicylic  acid, 
boracic  acid,  and  their  derivatives  may  be  mentioned. 


T  Lazarus,  Zeit.  f.  Hyg.,  8:  207. 


Principles  of  Milk  Preservation.  89 

Formalin  has  been  extensively  advertised  of  late,  but 
even  though  this  agent  does  not  exert  as  prejudicial  an 
effect  on  human  tissues  as  some  of  the  other  disinfectants, 
yet  it  must  be  regarded  with  the  remainder  of  chemical 
preservatives  as  undesirable. 

These  substances  can  be  added  to  milk  in  quantities 
not  recognizable  to  the  taste,  and  they  will  materially  in- 
crease the  time  that  milk  will  remain  sweet,  but  their 
general  use  in  milk  is  to  be  strongly  deprecated.  In 
many  European  countries  their  use  is  prohibited  entirely. 
A  large  number  of  the  preservatives  are  sold  under  pro- 
prietary names,  and  as  a  rule  for  extremely  high  prices, 
but  they  all  depend  for  their  efficiency  upon  the  anti- 
septic action  of  chemicals. 

87.  Detecting-  preservaline.  The  detection  of  these 
various  preservatives  in  milk  does  not  come  within  the 
general  province  of  this  work,  but  Farrington1  has 
suggested  such  a  simple  means  for  the  determination 
of  the  presence  of  preservaline  and  other  boracic  acid 
compounds  that  is  so  intimately  connected  with  the  de- 
velopment of  acid  that  it  is  given  here.  When  the  acidity 
of  a  normal  milk  reaches  0.3-0.4%,  it  tastes  sour,  but 
when  boracic  acid  is  added  to  fresh  milk,  the  acidity  is 
much  increased  without  affecting  the  taste.  In  fact,  for 
some  reason  not  yet  explained,  the  acidity  of  the  milk  is 
increased  considerably  more  than  that  due  to  the  chemical 
added.  Therefore  milks  having  a  high  acid  reaction  (ex- 
ceeding 0.3-0.4% )  and  not  tasting  perceptibly  sour  have  in 
all  probability  been  treated  with  preservatives.  Failure  to 
sour,  especially  when  milk  is  kept  at  room  temperatures 
for  several  days,  is  pretty  conclusive  evidence  that  chem- 
icals have  been  used.  The  sale  of  these  preparations  for 
use  in  milk  finds  its  only  outlet  with  those  dairymen  who 

1  Farrington,  Journ.  Amer.  Chem.  Soc.,  Sept.,  1896. 


90  Dairy  Bacteriology. 

are  anxious  to  escape  the  exactions  that  must  be  met  lay 
all  who  attempt  to  handle  milk  in  the  best  possible 
method.  Dirty  milk  drugged  with  these  chemicals  is  fre- 
quently able  to  falsely  pass  as  an  extra  good  product. 

88.  Physical  agents.     The  physical  methods  of  milk 
preservation  as  a  rule  are  much  more  feasible  than  the 
chemical,  as  they  do  not  injure  the  nutritive  value  of  the 
fluid  to  such  an  extent.    The  efficiency  of  physical  meth- 
ods of  preserving  milk  is  dependent  upon  the  elimination 
or  destruction  of  bacterial  life,  or  at  least  an  inhibition 
of  their  powers  of  growth.     Some  methods,  such  as  the 
use  of  electricity,  or  of  compressed  air,  or  different  gases 
have  been  suggested,  but  a  .practical  treatment  of  milk 
with  these  agents  has  not  as  yet  been  devised.     Different 
methods  of  nitration  have  been  tried.     Gravel  or  sand 
niters  have  been  recommended  for  purifying  milk- supplies. 
In  these  the  suspended  foreign  matter,  and  a  part  of  the 
bacteria   can  undoubtedly  be  removed,   but  the  incon- 
venience of  sterilizing  such  a  filter  would  seem  to  be 
considerable.     Filters  of  this  character  have,  however, 
been  satisfactorily  used  by  the  Copenhagen  Dairy  Co.,  for 
a  number  of  years.  Germ-proof  filters,  such  as  the  Pasteur 
filter  that  are  so  efficient  in  water  purification,  are  not 
applicable  to  milk  as  they  clog  so  quickly.     Recently  the 
use  of  cellulose  as  a  filtering  substance  has  been  proposed, 
and  according  to  Backhaus  and  Comlien,1  this  is  satis- 
factory from  a  bacteriological  as  well  as  a  mechanical 
stand-point. 

89.  Freezing-.     Chilling  the  milk  has   always  been 
practiced  as  a  means  of  enhancing  the  keeping  quality, 
but  recently  the  attempt  has  been  made  to  transport  milk 
for  long  distances  in  a  frozen  condition,  in  which  state 
bacterial  growth  is  entirely  suspended.     Milk  can  be  pre- 


1  Ber.  Landw.  Inst.  Univ.  Konigsberg,  2:  12,   1897. 


Principles  of  Milk  Preservation.  91 

served  best  by  first  pasteurizing  it,  then  freezing  it  and 
shipping  in  this  condition.1  Another  process  is  Casse's 
method  in  which  a  quantity  of  milk  is  frozen  and  placed 
in  large  cans.  The  intervening  space  is  then  filled  with 
fresh  milk  and  the  whole  closed  air  tight.  It  is  claimed 
this  milk  keeps  for  weeks  in  this  condition.2  One  of  the 
difficulties  of  simple  freezing  is  that  the  milk  constitu- 
ents separate  out  so  thoroughly  that  it  is  difficult  to  per- 
fectly re- incorporate  them  upon  melting. 

90.  Condensed  milks.     Milks  may  be  preserved  for 
an  indefinite  period  of  time  by  condensing  them.     The 
keeping  quality  of  such  milk  often  depends,  however,  upon 
the  action  of  another  principle,  viz.,  the  inhibition  of  bac- 
terial development  by  reason  of  the  concentration  of  the 
medium.      This  degree  of   concentration  is  reached  in 
either  of  two  ways,  by  the  addition  of  sugar,  or  by  evap- 
orating the  watery  solution  by  boiling  it  down  either  in 
open  air  or  preferably  in  a  vacuum  pan.     In  the  milks 
preserved  by  addition  of  sugar,  the  bacteria  are  not  nec- 
essarily destroyed  but  they  are  unable  to  grow  on  account 
of  the  concentrated  condition  of    the  medium.      Such 
milks  diluted  with  several  volumes  of  water,  even  sterile 
water,  not  infrequently  undergo  the  ordinary  decomposi- 
tion changes.     In  some  cases,  the  heating  process  also 
destroys  in  part  the  contained  germ  life. 

9 1 .  High  temperatures.     Heat  has  long  been  used  as 
a  preserving  agent.     Milk  has  been  scalded  or  cooked 
from  time  immemorial  to  keep  it.     Heat  may  be  used  at 
different    temperatures,   and  when  so  applied  exerts   a 
varying  effect,   depending  upon  temperature  employed. 
All  methods  of  preservation  by  heat  rest,  however,  upon 
the  use  of  the  two  following  principles: 

1  Milch  Zeit..,  No.  9,  1895. 

2 Milch  Zeit.,  No.  33,  p.  527,  1897. 


92 


Dairy  Bacteriology. 


1.  A  temperature  above  the  maximum  growing- point 
(42-45°  C.)  and  below  the  the  thermal  death-point  (52- 
68°  C.)  will  prevent  further  growth,  and  consequently 
fermentative  action. 

2.  A  temperature  above  the  thermal  death-point  de- 
stroys bacteria,  and  thereby  stops  all  changes.    This  tem- 
perature varies,  however,  with  the  condition  of  the  bac- 
teria being  much  higher  for  spore-bearing  species. 

Attempts  have  been  made  to  employ  the  first  principle, 
viz.,  prolonged  heating  above  growing  temperature,  but 
when  milk  is  so  heated,  its  physical  appearance  is  changed. 
The  methods  of  heating  most  satisfactorily  used  are 

known  as  sterilization 
and  pasteurization,  in 
which  a  degree  of  tem- 
perature is  used  approxi- 
mating the  boiling-  and 
scalding-points  respect- 
ively. 

92.  Characteristics 
of  heated  milk.  When 
milk  is  subjected  to  the 
action  of  heat,  a  num- 
ber of  changes  take  place 
in  its  reactions. 

1 .  Thinner  body .  Milk , 

FIG.  15.     Microscopic  appearance  of  nor- 

mal  milk.     The   fat  globules  are  grouped  but     more     especially 

together  into  tiny  clots.    The  consistency  cream     heated   to    a  tem- 
or  body  01  cream  is  in  part  due  to  this  char-  m 

acteristic.  perature  exceeding  150 

F.,  becomes  much  thin- 
ner, a  condition  due  to  a  change  in  the  grouping  of  the 
fat  globules.1     In  normal  milk,  the  butter-fat  is  massed 
together  in  microscopic  clots  as  in  fig.  15.     In  heated 
1  Babcock  &  Russell,  13th  Kept.  Wis.  Stat.,  p.  73,  1896. 


Principles  of  Milk  Preservation.  93 

cream  these  clumps  are  broken  down  and  the  globules 
are  homogeneously  distributed  as  in  fig.  16. 

2.  Cooked  flavor.     If  milk  is  heated  to  160°  F.,  it  ac- 
quires a  cooked  taste  that  becomes  more  pronounced  as 
the  temperature  is  further  raised.     The  cause  of  this 
change  is  not  well  known.    Usually  it  has  been  explained  ' 
as  being  produced  by  changes  in  the  nitrogenous  ele- 
ments   in  the   milk,  particularly  in  the  albumen.     Re- 
cently ,  Thoerner1  has  pointed  out  the   coincidence  that 
exists  between  the  appearance   of   a   cooked   taste  and 
the  loss  of  certain  gases  that  are  expelled  by  heating. 
He  finds  that  the  milk  heated  in  closed  vessels  from  which 
the    gas    cannot   escape 

has    a   much    less    pro 
nounced    cooked    flavoi 
than  if  heated  in  an  open 
vessel. 

3.  Fermentative 
changes.    Normally  milk 
undergoes  the  lactic  acid 
fermentation.     If, 
ever,  it  is    heated   to 
temperature    above 
thermal    death-point 
these  non-spore-bearing 
organisms,  as  is  the  case 

.,      -i  FIG.  16.     Microscopic  appearance  of  milk 

in  pasteurizing,  it  does  heated  to  150o  F  or  above<  The  aggrega. 

not   SOUr  but    CUrdleS   by  tionof ifat  globules  that  is  present  in  normal 

j,  ,      „  milk  is   broken  down,   the  globules    being 

means    Ot    a   rennet     ter-  homogeneously  distributed  throughout  the 

ment   Which     is     Secreted  mil^  serum.  This  lessens  the  consistency  of 

,         . ,  ,  , ,      ,  cream  and  makes  it  appear  thinner. 

by  those    bacteria    that 
resist  the  heating  process. 

4.  Action  toward  rennet.     The  action  of   heat  causes 
the  soluble  salts  of  lime  in  the  milk  to  be  precipitated,  and 

1  Thoerner,  Chem.  Ztg.,  18:  845. 


"94  Dairy  Bacteriology. 

as  the  curdling  of  milk  by  rennet  is  dependent  upon  the 
presence  of  these  salts,  their  absence  in  heated  milks  retards 
greatly  the  rennet  action .  The  character  of  the  coagulum 
is  also  considerably  different  from  what  it  is  with  raw 
milk,  being  much  softer  and  less  tenacious. 

5.  Hydrogen  peroxid  test  (Storch's  test).  When  a 
small  quantity  of  potassium  iodid  and  starch  is  added 
to  milk  and  the  same  then  treated  with  a  few  drops  of 
dilute  hydrogen  peroxid,  normal  milk  will  turn  blue, 
while  that  heated  to  175°  F.  or  above,  will  not  change. 
This  reaction  is  due  to  the  fact  that  the  inherent  enzymes 
normally  found  in  milk  are  destroyed  by  this  degree  of 
heat,  and  therefore  the  hydrogen  peroxid  is  not  decom- 
posed1 . 

93.  Sterilization.  To  thoroughly  sterilize  milk,  it  is 
necessary  to  render  it  germ  free.  So  far  as  the  vegetating 
bacteria  are  concerned,  this  can  be  done  by  an  applica- 
tion of  heat  such  as  is  used  in  pasteurizing,  but  on  ac- 
count of  the  presence  of  spore-bearing  forms,  a  much 
higher  temperature  is  necessary.  To  render  milk  per- 
fectly sterile,  it  is  necessary  to  heat  it  for  about  two 
hours  at  250°  F.,  or  thirty  minutes  at  270°  F.  At  the 
temperature  of  boiling- water,  milk  often  withstands,  es- 
pecially during  the  summer  months,  a  prolonged  steam- 
ing for  five  to  six  hours. 

Another  method  of  sterilizing  is  that  used  in  what  is 
k:nown  as  intermittent  sterilization,  a  method  employed 
by  Dahl,  of  Norway.  This  consists  in  heating  the  milk 
to  a  temperature  fatal  to  the  contained  bacteria,  158°  F. 
or  above,  for  one-half  hour  in  order  to  kill  the  vegetating 
bacteria.  The  milk  is  then  placed  under  conditions  that 
will  cause  the  latent  spores  to  germinate.  After  this  oc- 

1  V.  Storch,  4.0th  Kept.  Danish  Expt.  Stat.,  Copenhagen,  1898. 
Babcock  and  Russell,  15th  Kept.  Wis.  Expt.  Station,  p.  84,  1898. 


Principles  of  Milk  Preservation.  95 

curs,  a  second  application  of  heat  is  made  on  the  suc- 
ceeding day,  and  so  on  for  three  or  four  days.  By  this 
intermittent  process,  all  spores  are  given  a  chance  to 
germinate,  and  thus  become  susceptible  to  a  relatively 
low  heat. 

These  methods  which  necessitate  so  protracted  a  treat- 
ment are  manifestly  out  of  the  question  for  commercial 
purposes  unless  a  product  is  desired  that  will  keep  for  a 
considerable  period  of  time. 

When  milk  is  sterilized  commercially,  it  is  usually 
treated  for  a  single  time  at  a  high  temperature,  and  in 
some  of  the  better  types  of  apparatus  is  rendered  nearly 
sterile,  or  at  least  the  small  amount  of  germ-life  remain- 
ing is  so  weakened  by  the  treatment  given  that  develop- 
ment is  suspended,  unless  the  conditions  are  very  favor- 
able. In  most  cases  sterilized  milks  acquire  a  more  or 
or  less  pronounced  cooked  taste.  This  is  more  of  an  ob- 
jection in  America  than  in  Europe,  where  in  most  cases 
milk  is  usually  heated  before  being  consumed.  Appar- 
atus of  this  class  is  usually  expensive,  so  that  the  cost 
of  sterilized  milk  is  higher  than  the  normal  product. 
Machines  for  this  purpose  have  not  been  readily  adopted 
in  this  country,  the  tendency  being  to  employ  the  follow- 
ing process  for  milk  preservation. 

94.  Pasteurization.  In  this  method  the  degree  of 
heat  ranges  from  140°-175°  F.  and  the  application  is 
made  for  only  a  limited  length  of  time.  The  process  was 
first  extensively  used  by  Pasteur  (from  whom  it  derives 
its  name)  in  combatting  the  various  maladies  of  beer 
and  wine.  Its  importance  as  a  means  of  increasing  the 
keeping  quality  of  milk  was  not  generally  recognized 
until  a  few  years  ago ;  but  the  method  is  now  growing 
rapidly  in  favor  as  a  means  of  purifying  milk  for  com- 
mercial purposes.  The  method  does  not  destroy  all 


96  Dairy  Bacteriology. 

germ-life  in  milk;  it  affects  only  those  organisms  that 
are  in  a  growing,  vegetative  condition,  but  if  the  same  is 
quickly  cooled,  it  enhances  the  keeping  quality  very  ma- 
terially. 

The  experiments  of  Bitter1  indicate  that  when  stored 
at  86°  F.,  properly  pasteurized  milk  will  remain  sweet 
from  six  to  eight  hours  longer  than  raw  milk.  At  77° 
F.,  ten  hours;  at  73°  F.,  twenty  hours;  at  58°  F.,  from 
fifty  to  seventy  hours.  In  our  experience,  pasteurized 
cream  when  kept  in  an  ordinary  refrigerator  usually  re- 
mains sweet  for  a  period  of  four  to  six  days,  and  some- 
times even  longer. 

95.  Requirements  of  Pasteurized  milks.  Pasteur- 
ized products  should  possess  the  following  requirements : 

1.  Absolute  freedom   from    disease   bacteria.     Fortu- 
nately the  disease  bacteria  that  are  apt  to  be  transmitted 
by  means  of  the  milk   (tuberculosis,   typhoid,  etc.),  do 
not  form  spores.     Therefore,  a  proper  pasteurization  will 
destroy  the  virus  of  these  diseases. 

2 .  Ordinary  milk  bacteria  should  be  diminished .   Proper 
pasteurizing  will  destroy  all  non- spore-bearing  or  vegeta- 
tive bacteria. 

3.  Improved  keeping  quality.     This  destruction  of  the 
normal  milk  flora  will  result  in  lengthening  the  time  that 
milk  will  remain  sweet. 

4.  Normal  in  taste  and  appearance.     Pasteurized  pro- 
ducts should  have  no  perceptible   cooked  taste.     They 
will,  however  appear  thinner  in  consistency,  a  condition 
that  often  leads  to  the  belief  that  they  contain  less  fat. 
This  is  not  a  serious  objection  in  the  case  of  milk,  but  in 
cream  it  is  more  apparent.     It  can,  however,  be  easily 
remedied  by  the  use  of  viscogen  (98). 


1  Bitter,  Zeit.  f.  Hyg.,  8:  240,  1890. 


Principles  of  Milk  Preservation.  97 ' 

96.  Pasteurized  vs.  sterilized  milk.     For  consump- 
tion within  a  few  days,  pasteurized  products  are  as  well 
adapted  as  sterilized.    For  export  and  long- keeping  milk, 
sterilized    is    better.     Pasteurized   is  less   objectionable 
than  sterilized  on  account  of  cooked  taste  and  flavor.    As 
to   the  relative   digestibility,  authorities  disagree.     For 
the  healthy  child  there  is  probably  but  little  choice;  for 
infants,  pasteurized  is  believed  to  be  better  adapted.    Pas- 
teurized products  can  be  prepared  and  sold  at  less  ex- 
pense than  sterilized. 

97.  Children's   milk.     Attention  has   already  been 
called  to  the  relation  which  exists  between  infant  mor- 
tality and  the  use  of  cows'  milk.     The  cause  of  this  lies 
in  the  fact  as  shown  by  Soxhlet  that  breast  milk  is  con- 
sumed directly  and   in  a  condition   practically  sterile, 
although  the  researches  of  Knochensteirn  show  that  it  is 
quite  as  impossible  to  secure  germ  free  mother's  milk  as 
it  is  cow's  milk.     The  method  of  handling  cow's  milk  is 
such  that  germ  life  finds  ready  access  and  opportunity  to 
grow,  with  the  result  that  infantile  disturbances  are  com- 
mon where  ordinary  milk  is  used.     Soxhlet  introduced  a 
method  of  sterilizing  designed  for  family  use  that  has 
proven  very  successful  in  Germany,  where  it  is  widely 
used. 

In  sterilizing  or  pasteurizing  milk  for  infant  feeding, 
it  is  desirable  that  only  a  sufficient  quantity  for  each 
day's  use  be  prepared  at  once,  for  any  milk  that  is  not 
perfectly  sterile  will  gradually  develop  the  latent  germ 
life  that  is  in  the  same,  and  as  these  forms  belong  to  the 
peptonizing  class  of  organisms,  their  presence  is  not  de- 
sirable, as  has  been  shown  by  Fliigge1 . 


Fliigge,  Zeit.  f.  Hyg  ,  17:  272. 
7-B. 


98  Dairy  Bacteriology. 

98.  Restoration  of  "body"  of  pasteurized  cream. 

The  action  of  heat  causes  the  tiny  groupings  of  fat  glob- 
ules in  normal  milk  (fig.  15)  to  break  up,  and  with  this 
change  which  occurs  in  the  neighborhood  of  150°  F.,  the 
consistency  of  the  liquid  is  diminished,  notwithstanding 
the  fact  that  the  fat-content  remains  unchanged.  Bab- 
cock  and  the  writer  l  devised  the  following  ' '  cure J '  for 
this  apparent  defect.  If  a  strong  solution  of  cane-sugar 
is  added  to  freshly  slaked  lime  and  the  mixture  allowed 
to  stand,  a  clear  fluid  can  be  decanted  off.  The  addition 
of  this  alkaline  liquid,  which  they  call  "  viscogen,"  to 
pasteurized  cream  in  proportions  of  about  one  part  of 
sugar-lime  solution  to  100-150  of  cream,  restores  the  con- 
sistency of  the  cream,  as  it  causes  the  fat  globules  to 
cluster  together  in  small  groups. 

The  relative  viscosity  of  creams  can  be  easily  deter- 
mined by  the  following  method  (fig.  17) : 

Take  a  perfectly  clean  piece  of  glass  (plate  or  picture 
glass  is  preferable,  as  it  is  less  liable  to  be  wavy) .  Drop 
on  one  edge  two  or  three  drops  of  cream  at  intervals  of 
an  inch  or  so.  Then  incline  piece  of  glass  at  such  an 
angle  as  to  cause  the  cream  to  flow  down  surface  of  glass. 
The  cream  having  the  heavier  body  or  viscosity  will  move 
more  slowly.  If  several  samples  of  each  cream  are  taken, 
then  the  aggregate  lengths  of  the  different  cream  paths 
may  be  taken,  thereby  eliminating  slight  differences  due 
to  condition  of  glass. 

99.  Pasteurizing*  details.     While  the  •  pasteurizing 
process  is  exceedingly  simple,  yet  in  order  to  secure  the 
best  results,  certain  conditions  must  be  rigidly  observed 
in  the  treatment  before  and  after  the  heating  process.  It 
is  a  mistaken  idea  that  any  milk  is  fit  for  pasteurizing. 

1  Babcock  and  Russell,  Bull.  54,  Wis.  Expt.  Stat.,  also  13th  Kept. 
Wis.  Expt.  Stat,  p.  81,  1896. 


Principles  of  Milk  Preservation. 


99 


The  fresher  and  better  the  milk,  the  less  likely  it  is  to 
contain  deleterious  spore-bearing  bacteria  which  are  not 
destroyed  by  pasteurizing.  The  unhindered  development 
of  these  germs  in  the  milk  may  sometimes  give  rise  to  the 
production  of  decomposition  products  that  are  toxic  or 
at  least  highly  irritating  in  their  character1 . 


FIG.  17.    Relative  consistency  of  pasteurized  cream,  before  (.A)  and  after  (B 
treatment  with  viscogen.    Cream  flowing  down  inclined  glass  plate. 

1 00.  Selection  of  milk.  Milk  for  pasteurizing  should 
be  as  clean  as  possible,  for  in  general  terms,  the  bacteria 
associated  with  filth  and  dirt,  more  especially  manure 
particles,  belong  to  the  spore-bearing  putrefactive  class. 

1  Flugge,  Zeit.  f.  Hyg.,  17:  272. 


100  Dairy  Bacteriology. 

For  high  grade  pasteurizing,  it  is  advisable  to  purify  the 
milk  by  passing  it  through  a  separator,  unless  the  raw 
product  is  secured  under  conditions  that  exclude  the  ad- 
mission of  dirt. 

The  age  of  the  milk  and  the  conditions  under  which  it 
has  been  kept  should  also  be  carefully  noted.  Old  milk 
is  almost  always  richer,  not  only  in  bacterial  germs  but 
in  the  latent  spore  forms  as  well;  so  the  fresher  the  milk 
the  fewer  bacteria  it  will  have,  and  therefore  the  pasteur- 
izing process  will  be  the  more  complete. 

101.  Selecting"  milk  by  acid  test.  The  true  stand- 
ard for  selecting  milk  for  pasteurization  should  be  to  de- 
termine the  actual  number  of  bacterial  spores  that  are 
able  to  resist  the  heating  process,  but  this  method  is  im- 
practicable under  commercial  conditions. 

The  following  method  while  only  approximate  in  its 
results  will  be  found  helpful.  Assuming  that  the  age  or 
treatment  of  the  milk  bears  a  certain  relation  to  the  pres- 
ence of  spores,  and  that  the  acid  increases  in  a  general 
way  with  an  increase  in  age  or  temperature,  the  amount 
of  acid  present  may  be  taken  as  an  approximate  index  of 
the  suitability  of  the  milk  for  pasteurizing  purposes. 
Biological  tests  were  carried  out  in  the  author's  labora- 
tory1 on  milks  having  a  high  and  low  acid  content,  and 
it  was  shown  that  the  milk  with  the  least  acid  as  a  rule 
was  the  freest  from  spore-bearing  bacteria. 

This  acid  determination  can  be  made  at  the  weigh-can 
by  employing  the  Farrington  alkaline  tablet  which  is 
used  in  cream-ripening.  Where  milk  is  pasteurized  under 
general  creamery  conditions,  none  should  be  used  con- 
taining more  than  0.2%  acidity.  If  only  perfectly  fresh 
milk  is  used  the  amount  of  acid  will  generally  be  about 
0.15%  with  phenolphthalein  as  indicator. 

i  Shockley,  Thesis,  Univ.  of  Wis.,  1896. 


Principles  of  Milk  Preservation. 


101 


Circumstances  may  arise  that  might  lead  to  an  error  if 
this  method  is  blindly  followed.  It  has  been  pointed 
out  that  if  milk  is  allowed  to  stand  in  rusty  cans  for 
some  time  its  acid  content  is  diminished  materially.  Bid- 
dick1  has  reported  the  following  interesting  observations. 
The  average  of  nine  samples  of  milk  brought  to  a  factory 
in  clean  cans  contained  .228%  acidity,  while  that  of  nine 
other  patrons  brought  in  rusty  cans  contained  only  .134% 
acid.  Milk  kept  in  rusty  cans  is  sure  to  contain  large 
quantities  of  bacteria,  even  though  its  acid  content  may 
be  low. 


&  OunceBottle.  Measure 


FIG.  18.     Apparatus  used  in  making  rapid  acid  test. 

102.  Making-  the  acid  test.  Fig.  18  gives  the  ap- 
paratus necessary  to  make  an  approximate  determination 
of  the  acidity  of  the  milk.  A  solution  of  the  alkaline 
tablets  is  first  prepared  by  dissolving  same  in  clean  soft 
water,  one  tablet  for  each  ounce  of  water;  thus  eight 

1  Biddick,  Hoard's  Dairyman,  July  30,  1897. 


/102  Dairy  Bacteriology. 

tablets  to  an  eight  ounce  bottle  of  water.  In  determin- 
ing the  acidity  in  each  patron's  milk,  a  number  of  com- 
mon white  cups  are  used,  one  for  each  patron.  Two 
measures  full  of  the  alkali  solution  are  placed  in  each  cup, 
and  then  as  the  milk  is  received  at  the  weigh- can,  one- 
half  as  much  milk  is  added  to  the  alkali  solution  in  the 
cup,1  and  the  whole  gently,  but  thoroughly  shaken.  If 
the  pink  color  of  the  alkali  solution  persists  even  faintly, 
it  shows  that  there  is  not  enough  acid  in  the  milk  to  neu- 
tralize the  same;  if  it  disappears  altogether,  leaving  the 
milk  white  in  color,  it  indicates  that  there  is  more  acid 
•  in  the  milk  than  can  be  neutralized  by  the  alkali  of  the 
tablet  solution.  Any  standard  desired  can  be  chosen,  but 
where  the  relation  of  the  milk  to  the  alkali  solution  is 
maintained  in  above  ratio  (two  to  one),  it  indicates 
that  0.2%  acidity  is  present,  if  the  alkali  is  completely 
neutralized.  Extended  experience  has  shown  the  neces- 
sity of  selecting  milks  for  pasteurizing  that  come  within 
this  standard. 

103.  Temperature  and  time  limits.  The  time  and 
temperature  limits  in  pasteurizing  are  subject  to  consid- 
erable variation.  The  minimum  temperature,  however, 
must  exceed  the  thermal  death-point  at  which  the  milk 
bacteria  are  destroyed.  As  the  tubercle  bacillus  is  some- 
times found  in  milk,  and  as  it  is  one  of  the  most  resist- 
ant organisms  in  its  vegetative  state  that  is  known,  the 
thermal  death-point  of  this  germ  serves  as  a  minimum 
standard  for  efficient  pasteurizing. 

The  determination  of  this  point  depends  on  the  follow- 
ing conditions: 

1.  The  temperature  of  heat  used. 

2.  The  length  of  exposure  to  the  heat. 

1  Farrington,  Wis.  Expt.  Stat.,  Bull.  52. 


Principles  of  Milk  Preservation.  103 

With  either  condition  fixed,  the  same  result  is  accom- 
plished more  rapidly  by  an  increase  either  in  temperature 
or  duration  of  exposure,  so  that  an  exposure  for  a  longer 
time  at  a  lower  temperature  is  quite  as  effective  as  a 
shorter  exposure  at  a  relatively  higher  temperature. 

With  the  great  majority  of  bacterial  species  that  have 
been  individually  tested  as  to  their  thermal  death-point, 
140°  F.  for  ten  minutes  has  been  found  fatal.  This  de- 
gree of  heat  suffices  to  kill  all  the  disease-producing  bac- 
teria that  are  found  in  milk,  with  the  exception  of  the 
tubercle  bacillus. 

According  to  Forster,  heating  thirty  minutes  at  149°  F. , 
fifteen  minutes  at  155°  F.,  or  ten  minutes  at  167°  F. 
suffices  to  destroy  this  germ. 

The  maximum  temperature  that  can  be  employed  is  just 
below  the  point  at  which  the  milk  will  permanently  ac- 
quire a  cooked  flavor.  The  appearance  of  this  peculiar 
flavor  cannot  be  detected  with  absolute  accuracy,  but  it 
is  not  far  from  158°  F.  It  appears  in  milk  to  some  ex- 
tent before  this  temperature  is  reached  but  upon  chilling 
disappears.  It  is  not  enough  to  heat  milk  to  this  tem- 
perature, but  it  should  be  maintained  for  the  requisite 
length  of  time. 

1 04.  Subsequent  chilling".  The  heating  process  de- 
stroys only  the  vegetative  organisms,  the  spores  resisting 
this  temperature.  To  prevent  the  germination  of  these 
latent  forms  the  milk  should  be  quickly  chilled,  for  if  ger- 
mination once  occurs,  they  can  develop'  at  even  low  tem- 
peratures. 

The  following  experiments  by  Marshall1  are  of  interest 
as  showing  the  influence  of  refrigeration  on  germination 
of  spores. 

1  Marshall,  Mich.  Expt.  Stat.,  Bull.  147,  p.  47. 


104 


Dairy  Bacteriology. 


Cultures  of  organisms  that  had  been  isolated  from 
pasteurized  milk  were  inoculated  into  bouillon.  One  set 
was  left  to  grow  at  room  temperature,  another  was  pas- 
teurized and  allowed  to  stand  at  same  temperature, 


FIG.  19.  Diagram  showing  temperature  changes  in  pasteurizing,  and  the  re- 
lation of  same  to  bacterial  growth. 

Shaded  zone  represents  limits  of  bacterial  growth,  10-43°  C.  (50-109°  F.);  the  in- 
tensity of  shading  indicating  rapidity  of  development.  The  solid  black  line 
shows  temperature  of  milk  during  the  process.  The  necessity  for  rapid  cooling 
is  evident  as  the  milk  falls  in  temperature  to  that  of  growing  zone. 

while  another  heated  set  was  kept  in  a  refrigerator. 
The  unheated  cultures  at  room  temperature  showed  evi- 
dence of  growth  in  thirty  trials  in  an  average  of  26 


Principles  of  Milk  Preservation. 


105 


hours;  29  heated  cultures  at  room  temperature  all  de- 
veloped in  an  average  of  50  hours,  while  the  heated 
cultures  kept  in  refrigerator  showed  no  growth  in  45 
days  with  but  four  exceptions. 

105.  Preparation  of  utensils.    After  the  milk  is 
pasteurized,  it  must  of  necessity  be  stored  and  handled 


H.S. 


S.TP 


FIG.  20.  Steam  sterilizer  for  sterilizing  utensils  (cans,  bottles,  etc.).  St.  p., 
steam  pipe;  St.  p.  v.,  vent  for  steam  pipe;  w.  s.,  wire  shelf;  c,  outlet  cock  for 
condensed  water. 

in  germ  free  receptacles.  All  utensils  such  as  cans,  dip- 
pers, bottles,  etc.,  must  be  thoroughly  sterilized.  For 
this  purpose  a  sterilizing  oven  should  be  had  which  is 
steam  fitted,  so  that  direct  steam  can  be  used.  Material 
of  this  sort  after  being  thoroughly  cleansed,  should  be 
steamed  for  one-half  to  three-quarters  of  an  hour.  Ster- 
ilized bottles  should  be  kept  protected  from  dust  until 
they  are  used. 


106 


Dairy  Bacteriology. 


106.  Bottling"  and  handling- the  product.     In  bot- 
tling the  product  it  is  necessary  to  keep  the  milk  pro- 
tected from  reinfection.     It  may  be  bottled  from  a  large 
can  with  a  bottom  faucet,  or,  on  a  large  scale,  with  com- 
mercial bottling  machines  that  fill  several  bottles  at  once. 
If  "viscogen"  is  added  to  restore  consistency  of  cream, 
it  should  be  done  before  bottling,   but   not  before  the 
cream  is  thoroughly  cooled.      The  best  bottles  for   the 
purpose  are  those  that  have  a  plain  pulp  cap.     All  metal 
fastenings   or   stoppers  are  dirt  catchers  and  are  likely 
to  get  out  of  order.     It  is  bur  practice  to  heat  pulp  caps 
in  paraffin,  thereby  rendering  them  more  pliable  and  at 
the  same  time  sterilizing  them.     Bottles  sealed  with  hot 
caps  in  this  way  are  tightly  closed. 

In  delivering  pasteurized  products,  it  is  always  neces- 
sary to  use  care  in  handling  to  prevent  the  cream  and 
milk  from  being  warmed  up,  and 
thus  inciting  into  activity  the  latent 
spores. 

107.  Pasteurizing1  apparatus. 
If  it  is  desired  to  pasteurize  milk 
or  cream  for  direct  consumption, 
the  treatment  differs  somewhat  from 
that  when  pasteurizing  for  butter- 
making  is  the  object  in  view.     The 
equipment  necessary  for  the    first 
type  of  pasteurizing  may  be  divided 
into  two  general  classes. 

1.  Apparatus  of  limited  capacity 
designed  for  private  family  use. 

2.  Apparatus  of  sufficient  capac- 
ity to  pasteurize  on  a  commercial  scale. 

108.  Domestic  apparatus.     Pasteurization   can  be 
easily  and  efficiently  done  in  a  limited  way  with  the  addi- 


FIG.  21.  Sectional  view 
of  family  pasteurizer.show- 
ing  milk  bottles  immersed 
in  water. 


Principles  of  Milk  Preservation. 


107 


tion  of  an  ordinary  dairy  thermometer  to  the  common 
utensils  found  in  any  kitchen.  Fig.  22  indicates  a  sim- 
ple contrivance  than  can  be  readily  arranged  for  this 
purpose. 


FIG.  22.     A  home-made  pasteurizer. 

The  following  suggestions  indicate  the  different  steps 
of  the  process: 

1.  Use  only  fresh  milk  for  this  purpose. 

2.  Place  milk  in  clean  bottles  or  fruit  cans,  filling  to  a 
uniform  level.     If  pint  and  quart  cans  are  used  at  the 
same  time,  an  inverted  dish  or  piece  of  wood  will  equal- 
ize the  level.     Set  these  in  a  flat-bottomed  tin  pail  and 
fill  with  warm  water  to  same  level  as  milk.     An  inverted 
pie  tin  punched  with  holes  will  serve  as  a  stand  on  which 
to  place  the  bottles  during  the  heating  process. 

3.  Heat  water  in  pail  until  the  temperature  of  same 
reaches  160°  F. ;   then  remove  from  source  of  direct  heat, 


108  Dairy  Bacteriology. 

cover  with  a  cloth  or  tin  cover  and  allow  the  whole  to 
stand  for  half  an  hour. 

4.  Remove  bottles  of  milk  and  cool  them  as  rapidly 
as  possible  without  danger  to  bottles  and  store  in  a  re- 
frigerator. 

109.  Apparatus    for    commercial    pasteurizing1. 
Pasteurizing  as  applied  to  the  preservation  of  milk,  orig- 
inated in  Germany  and  Denmark,  where  it  is  used  largely 
in  the  treatment  of  skim-milk,  and  the  heating  of  cream 
in  butter- making;     The  type  of  machinery  that  has  been 
devised  for  this  purpose  has  therefore  naturally  been  con- 
structed  on  somewhat  different   principles    than  would 
have  been  employed  where  milk  is  treated  for  direct  con- 
sumption.    For  the  treatment  of  milk  for  direct   con- 
sumption, it  is  believed  that  the  sanitary  phase  of  the 
problem  demands  more  attention  than  the  economic  oper- 
ation of  the  apparatus,  and,  therefore,  it  is  more  neces- 
sary to  use  a  machine  that  pasteurizes  so  thoroughly  that 
all  possible  disease  bacteria  are  destroyed,  than  it  is  to 
secure  apparatus  that  will  permit  of  the  handling  of  large 
quantities  of  milk.     Pasteurizing  involves  considerable 
time  and  trouble,  and  it  is  better  not  to  have  the  process 
done  at  all  than  to  have  it  imperfectly  performed. 

The  various  types  of  machinery  that  have  been  sug- 
gested for  this  use  may  be  grouped  as  follows,  depending 
upon  their  method  of  operation1. 

1.  Continuous  flow  machines. 

2.  Intermittent  machines. 

110.  Continuous  flow  pasteurizers.    Apparatus  of 
this  class  varies  much  in  detail,  but  they  possess  this 
common  principle  that  the  milk  enters  the  machine  in  a 

1  For  the  detailed  description  of  pasteurizing  machinery,  reference 
should  be  made  to  Monrad's  Pasteurization  of  Milk,  or  Weigmann's 
Conservierung  der  Milch. 


Principles  of  Milk  Preservation. 


109 


continuous  stream  and  is  generally  discharged  in  the 
same  way.  The  objection  to  this  type  of  apparatus  is 
that  the  time  of  heating  cannot  be  regulated  with  any 
certainty,  although  the  temperature  can  be  controlled 


FIG.  24.  Hochmuth's  combined  heater  and  cooler.  Cold  water  enters  at  k  and 
circulates  through  coil,  cooling  the  milk  which  is  heated  in  upper  part  of  ribbed 
surface.  This  warm  water  is  used  to  warm  the  milk,  and  then  steam  is  intro- 
duced through  d. 

in  part  by  varying  the  speed  of  flow.  Another  objection 
is  that  the  rapid  heating  often  necessitated  by  these  ma- 
chines scalds  the  proteids  of  the  milk,  making  them  ad- 
here to  the  walls  of  the  pasteurizer. 


110 


Dairy  Bacteriology. 


In  some  of  these  machines  (Thiel,  Kuehne,  Lawrence, 
De  Laval,  and  Hochmuth),  a  ribbed  surface  is  employed 
over  which  the  milk  flows,  while  the  opposite  surface  is 
heated  with  hot  water  or  steam.  Monrad,  Lefeldt  and 
Lentsch,  employ  a  centrifugal  apparatus  in  which  a  thin 
layer  of  milk  is  heated  in  a  revolving  drum. 


FIG.  25.  Monrad's  centrifugal  pasteurizer.  Milk  is  introduced  into  center 
through^,  and  is  thrown  out  on  the  walls  of  centrifuge  d,  the  steam  being  ap- 
plied to  the  outside  of  the  revolving  drum.  Reservoir  m  catches  and  holds  the 
milk  for  a  moment.  Milk  finally  flows  at  h. 

Some  of  these  machines  are  suitable  for  heaters,  if  the 
milk  was  held  in  vats  that  would  retain  the  temperature 
at  a  pasteurizing  point  for  a  sufficient  length  of  time. 

Other  machines  of  the  continuous  flow  type  employ  a 


Principles  of  Milk  Preservation.  Ill 

reservoir  that  is  filled  with  milk  and  surrounded  with  an 
outer  shell  that  contains  the  heating  agent,  steam  or  hot 
water. 

Some  of  this  class  are  provided  with  an  agitator  in  the 
milk  reservoir  so  as  to  hasten  the  equalization  of  tem- 
perature in  the  inner  chamber,  and  at  the  same  time  keep 
the  milk  in  motion  in  order  to  prevent  the  coagulation  of 
the  proteids. 

Most  of  them  are  arranged  for  a  continuous  delivery, 
the  milk  flowing  in  at  the  lower  end  and  displacing  that 
already  pasteurized,  which  flows  out  above  into  a  cooler. 
In  some  of  them  the  agitator  even  mixes  the  fresh  supply 
with  that  which  has  already  been  heated,  so  that  the  effi- 
ciency of  the  process  where  milk  is  being  treated  for 
direct  consumption,  is  much  lessened. 

Where  it  is  constructed  for  continuous  delivery,  the 
length  of  exposure  must  necessarily  be  quite  limited,  and 
as  the  temperature  of  the  milk  ought  not  to  exceed  160° 
F.  for  fear  of  scalding,  it  very  often  happens  that  the 
pasteurizing  process  is  not  efficient.  In  the  case  of  the 
sour  milk  organisms,  from  the  hygienic  standpoint,  it  is 
of  little  moment,  but  to  insure  absolute  freedom  from 
disease  germs,  the  temperature  and  the  time  of  exposure 
must  be  thoroughly  under  control.  Very  few  of  the  ma- 
chines intended  for  this  purpose  have  been  subjected  to 
a  rigid  bacteriological  test,  and  the  lack  of  this  has  allowed 
the  introduction  of  many  designs  that  may  be  adapted 
for  the  pasteurization  of  by-products  intended  for  animal 
food,  for  which  purpose  many  of  them  were  originally 
designed,  but  they  certainly  do  not  deliver  a  product  that 
can  be  relied  upon  for  human  food. 

111.  Intermittent  pasteurizers.  Inasmuch  as  the 
biological  and  physical  requirements  as  to  pasteurizing, 
necessitate  milk  being  heated  between  the  temperatures 


112  Dairy  Bacteriology. 

of  about  145°-160°  F.,  it  is  desirable  that  the  temperature 
should  be  under  absolute  control.  Moreover  the  time  limit 
must  also  be  known.  This  requires  the  treatment  of  a 
definite  quantity  of  material  for  a  definite  length  of  time 
at  a  definite  temperature.  A  fulfillment  of  these  condi- 
tions necessitates  the  use  of  the  intermittent  type  of 
apparatus,  or  continuous  apparatus  arranged  so  as  to 
practically  conform  to  the  discontinuous  process. 

The  simplest  way  in  which  these  conditions  can  be  car- 
ried out  is  to  employ  a  number  of  shot-gun  cans  immersed 
in  a  tank  of  hot  water.  By  means  of  this  crude  device 
milk  or  cream  can  be  pasteurized  more  effectually  than 
in  many  of  the  specially  designed  pieces  of  apparatus. 
Tanks  surrounded  with  water  spaces  can  also  be  used 
quite  successfully,  although  from  a  commercial  point  of 
view  apparatus  of  this  sort  is  imperfect,  unless  it  is  spe- 
cially designed  for  this  purpose. 

The  use  of  the  Boyd  cream  ripening  vat  has  been  sug- 
gested, and  this  fulfills  the  necessary  conditions  as  to  a 
commercial  pasteurizer.  The  cream  in  this  is  heated  by 
means  of  a  swinging  coil  immersed  in  the  same,  through 
which  hot  water  circulates. 

In  some  of  the  pasteurizers,  steam  is  introduced  di- 
rectly into  the  milk  or  cream,  as  in  Bentley's  apparatus. 
It  is  obvious  that  while  such  a  method  may  be  a  cheaper 
way  in  which  to  heat  the  milk,  still  the  proteids  of  the 
fluid  must  be  scalded  in  part,  although  the  temperature 
of  the  whole  mass  may  not  exceed  the  proper  pasteur- 
izing point. 

The  writer1  in  1894  devised  a  tank  pasteurizer  that  was 
made  to  conform  to  the  bacteriological  requirements.  It 
consists  of  a  long,  narrow  vat,  surrounded  by  a  water 
chamber,  on  the  bottom  of  which  is  placed  a  row  of  per- 

1  Russell,  Wis.  Expt.  Stat.,  Bull.  44. 


Principles  of  Milk  Preservation. 


113 


f orated  steam  pipes.  To  facilitate  the  heating  of  the 
milk,  both  the  milk  and  water  reservoirs  are  supplied 
with  agitators  having  a  to  and  fro  movement.  The  milk 
stirrers  are  perforated  so  as  to  give  an  up  and  down 


FIG.  26.    Russell's  pasteurizing  vat. 

movement,  thus  preventing  the  cream  from  rising  to  the 
top  of  the  vat  during  the  operation.  The  milk  is  with- 
drawn from  the  vat  by  means  of  a  stop- cock  that  is 
placed  inside  of  the  water  chamber.  This  cock  has  a 
circular  bore  so  that  when  open,  the  outlet  tube  presents 
a  continuous  passage  that  can  be  easily  and  thoroughly 
cleaned.  Placing  the  stop-cock  within  the  water  chamber 
prevents  the  accumulation  of  unpasteurized  milk  in  the 
outlet  tube  at  a  point  not  reached  by  the  heated  water, 
and  where  it  would  contaminate  the  whole  vat  when  the 
milk  was  withdrawn. 

The  Potts  pasteurizer  is  another  machine  of  the  inter- 
mittent type  that  has  recently  been  introduced  that  con- 
forms to  the  necessary  biological  conditions.  This  ap- 
paratus has  a  central  milk  chamber  that  is  surrounded 
with  an  outer  shell  containing  hot  water.  The  whole 
machine  revolves  on  a  horizontal  axis,  and  the  cream  or 
milk  is  thus  thoroughly  agitated  during  the  heating  pro- 
cess. 
8— B. 


114  Dairy  Bacteriology. 

112.  Coolers.  A  speedy  cooling  of  the  heated  pro- 
duct is  essential  to  success  in  pasteurizing.  Some  of  the 
machines  have  been  devised  for  a  combination  purpose, 
being  used  for  the  heating  and  subsequent  cooling  of  the 


FIG.  27.    Pott's  pasteurizer. 

milk.  This  is  an  evident  advantage  in  some  ways,  as  it 
lessens  the  amount  of  apparatus  necessary,  also  the  work 
involved  in  cleaning  the  same,  but  at  the  same  time  the 
problems  of  quick  heating  and  cooling  involve  somewhat 
different  principles,  so  that  for  the  most  economical 
manipulation  of  the  product,  separate  pieces  of  appara- 
tus are  advisable  where  the  business  warrants  such  ex- 
pense. 

The  simplest  method  of  treatment  in  cooling  is  to  draw 
off  the  milk  in  shot-gun  cans  and  place  these  first  in 
water,  then  in  ice -water. 

To  cool  milk  most  economically,  two  coolers  should  be 
provided.  One  of  these  can  use  cold  water,  and  by  the 
aid  of  good  well  water,  the  temperature  can  be  reduced 
to  nearly  that  of  the  water  in  a  short  time.  In  order, 
however,  to  lower  the  temperature  below  a  point  where 
spore  germination  will  readily  occur,  milk  should  be 
chilled  by  the  aid  of  ice.  This  may  be  applied  in  the 
same  cooler  as  is  used  for  running  cold  water,  by  sup- 
plying ice- water  for  the  latter  part  of  the  cooling  process. 


Principles  of  Milk  Preservation.  115 

To  use  ice  economically,  the  ice  itself  should  be  applied 
as  closely  as  possible  to  the  milk  to  be  cooled,  for  the 
larger  part  of  the  chilling  value  of  ice  comes  from  the 
melting  of  the  same.  To  convert  a  pound  of  ice  at  32° 


FIG.  28.  Water  cooler,  using  either  running  cold  water,  or  water  and  crushed 
ice.  The  milk  is  introduced  at  i.  m.  and  is  spread  out  in  m.  c.  in  a  thin  cylindri- 
cal sheet,  flowing  out  at  o.  m.  Cold  water  circulates  through  inside  of  milk 
cylinder  in  w.  c.,  while  ice  may  be  used  in  outer  water  chamber,  o.  w.  c. 

F.  into  a  pound  of  water  at  the  same  temperature  re- 
quires as  much  heat  as  would  suffice  to  raise  142  pounds 
of  water  one  degree  F.,  or  one  pound  of  water  142°  F. 
The  absorptive  capacity  of  milk  for  heat  is  not  quite  the 
same  as  it  is  with  water.  It  fluctuates  with  the  amount 
of  solids  in  the  milk,  but  for  ordinary  milk  is  about  .85 
while  water  is  taken  as  a  standard  1.0.  Hot  milk  would 
therefore  require  somewhat  less  ice  to  cool  it  than  would 
be  required  by  an  equal  volume  of  water  at  same  tem- 
perature. 

In  the  mere  melting  of  a  pound  of  ice,  a  large  part  of 
the  heat  in  a  pound  of  pasteurized  milk  will  be  absorbed. 
To  take  advantage  of  this,  the  ice  should  be  brought  in 
close  contact  with  the  milk  rather  than  to  spend  all  of 
this  absorptive  capacity  on  cooling  water  which  is  later 
applied  to  the  milk.  If  broken  ice  is  used  directly,  it 
should  be  arranged  so  that  the  milk  surrounds  it,  as  in 
this  way  the  specific  capacity  for  heat  that  is  latent  in 
the  ice  acts  on  the  milk  instead  of  being  radiated  in  part 
to  the  outside. 


116  Dairy  Bacteriology. 

113.  Bacteriological  study  of  pasteurized  milk. 

An  extended  bacteriological  examination  of  milk  and 
cream  pasteurized  on  a  commercial  scale  in  the  Russell 
vat  at  the  Wisconsin  Dairy  school  showed  that  over  99.8 


FIG.  29.  Effect  of  pasteurizing  on  germ  content  of  milk.  Black  square  repre- 
sents bacteria  of  raw  milk;  small  white  square,  those  remaining  after  pasteuri- 
zation. 

%  of  the  bacterial  life  in  raw  milk  or  cream  was  destroyed 
by  the  heat  employed,  i.  e.,  155°  F.  for  twenty  minutes 
duration1 .  In  nearly  one-half  of  the  samples  of  milk,  the 
germ  content  in  the  pasteurized  sample  fell  below  1,000 
bacteria  per  cc.,  and  the  average  of  twenty-five  samples 
contained  6 , 140  bacteria  per  cc .  In  cream  the  germ  content 
was  higher,  averaging  about  25 , 000  bacteria  per  cc .  When 
the  high  initial  content  of  either  milk  or  cream  is  taken 
into  consideration,  it  indicates  that  the  spore-bearing 
bacteria  are  relatively  few  in  properly  selected  milk  that 

1  Russell,  12th  Wis.  Expt.  Stat.  Kept.,  p    160,  18(J5. 


Principles  of  Milk  Preservation.  117 

is  efficiently  pasteurized.  The  kinds  that  are  destroyed 
are  mainly  the  lactic  acid  species;  those  that  resist  the 
pasteurizing  heat  are  generally  of  the  enzyme-forming 
class,  which  in  many  cases  are  also  able  to  liquefy  gelatin. 
The  acidity  of  pasteurized  products  always  increases  some- 
what, but  even  when  it  is  curdled,  it  rarely  contains  over 
0.3-0.4  %  of  acid.  The  organisms  that  remain  belong 
then  to  the  sweet  curdling  type  of  bacteria. 

While  these  organisms  capable  of  resisting  the  pasteur- 
izing temperature  are  fermentative  germs,  it  is  also  im- 
portant to  determine  if  they  have  the  power  of  forming 
toxic  substances  that  exert  a  harmful  effect  on  the  ani- 
mal body.  Fluegge1  has  found  several  forms. in  sterilized 
milk  that  are  able  to  produce  highly  toxic  substances. 
A  similar  study  has  been  made  under  the  writer's  direc- 
tion on  the  bacteria  isolated  in  pasteurized  milk  under 
commercial  conditions.  Shockley2  studied  thirteen  differ- 
ent forms  found  in  pasteurized  milk,  but  all  of  them  with 
a  single  exception  failed  to  cause  any  disturbance  when 
inoculated  into  white  mice  and  rabbits.  In  the  single 
instance,  fatal  results  were  only  obtained  when  large 
quantities  were  inoculated. 

1  Fluegge,  Zeit.  f.  Hyg.,  17:  272,  1894. 

2  Shockley,  Thesis,  Univ.  of  Wis.,  1896. 


PART  III. 

RELATION  OF  BACTERIA  TO  MILK 
PRODUCTS. 

CHAPTER  VIII. 

BACTERIA    IN    CREAM    AND    FACTORY 
BY-PRODUCTS. 

114.  Bacteria  in  cream  due  to  mechanical  causes. 

Cream  whether  secured  by  the  gravity  process  or  by  a 
cream  separator  is  invariably  richer  in  bacteria  than  the 
skim-milk  of  the  same  age.  A  sample  of  milk  might 
contain  less  than  100,000  germs  per  cc.  in  the  skimmed 
part,  while  the  bacterial  contents  of  the  cream  layer  in 
the  same  would  be  several  millions  for  the  same 
unit  of  volume.  This  is  largely  due  to  the  filtering 
out  of  the  microbes  during  the  process '  of  cream- 
ing. It  is  a  well-known  fact  in  sewage  filtration  that 
the  addition  of  some  chemical  that  will  cause  the  pre- 
cipitation of  certain  organic  elements  always  present  in  the 
sewage,  will  take  out  the  majority  of  bacteria  in  the  set- 
tling of  thispercipitate.  The  same  principle  seems  to  be 
operative  in  the  process  of  creaming,  except  that  the 
fat  globules  instead  of  sinking  to  the  bottom  rise  to  the 
surface.  Gravity-raised  cream  is  usually  richer  in  bac- 
teria than  separator  cream,  because  it  is  materially  older 
when  gathered. 

In  full  milk  separated  by  the  centrifugal  method,  there 
are  three  well-marked  products, — the  skim-milk,  the 
cream,  and  the  slime  that  adheres  to  the  separator  bowl. 

[118] 


Cream  and  Factory  By-Products.  119 

The  bacterial  content  of  each  of  these  will  vary  materially 
in  different  cases. 

The  slime  which  is  composed  of  particles  having  a 
greater  specific  gravity  than  the  milk  serum  is  thrown  out 
on  the  edge  of  the  revolving  fluid  by  centrifugal  force. 
This  material,  if  examined  microscopically,  will  be  found 
to  contain  large  quantities  of  foreign  matter  as  well  as 
innumerable  bacteria.  The  fact  that  it  rapidly  undergoes 
decomposition  is  evidence  of  its  high  germ  content. 

The  cream  will  almost  always  contain  a  larger  number 
of  bacteria  than  the  skim-milk.  Popp  and  Becker  found 
in  a  sample  of  whole  milk  containing  73,000  germs  per  cc. , 
the  following  germ  contents  after  it  had  been  separated: 
Cream,  58, 275  germs;  skim-milk,  21,700  germs,  and  the 
separator  slime  43, 900  per  cc.  This  centripetal  niovement 
of  a  large  number  of  germs  with  the  cream  indicates  that 
they  adhere  to  the  tiny  fat  globules,  for  this  peculiarity 
in  distribution  can  hardly  be  explained  on  the  ground  of 
their  specific  gravity,  as  they  remain  in  the  skim-milk  in 
considerable  numbers  in  spite  of  the  great  centrifugal 
pressure . 

115.  Tubercle  bacillus  and  separator  slime.  Ac- 
cording to  Scheurlen1  and  Bang2,  tubercle  bacilli,  if 
present  in  a  milk  are  largely  thrown  out  with  the  slime 
in  the  separating  process.  Moore3  found  in  milk  arti- 
ficially infected  with  tubercle  bacilli  that  the  separating 
process  diminished  them  to  such  an  extent  that  they 
could  not  be  determined  microscopically,  but  when  this 
separated  milk  was  inoculated  into  guinea-pigs,  infection 
occurred.  This  indicates  that  while  the  removal  was  con- 
siderable, yet  it  was  not  complete  enough  to  justify  the 

1  Scheurlen,  Arb.  a.  d.  k.  Ges.  Amte,  7:  269,  1891. 

2  Bang,  Land.  Woch.  f.  Sch.  Hoi.,  1894,  p.  47. 

3  Moore,  Year-book  of  U.  S.  Dept.  of  Agri.  1895,  p.  432. 


120  Dairy  Bacteriology. 

use  of  this  method  for  the  purification  of  infected  milk. 
Coupled  with  this  peculiar  relation  of  the  tubercle  germ 
to  centrifuge  slime,  is  the  fact  that  tuberculosis  among 
swine  is  much  more  prevalent  in  Denmark  and  North 
Germany  where  the  centrifugal  process  in  creaming  is 
extensively  used,  and  where,  until  recently,  the  swine  were 
fed  the  uncooked  separator  slime.  Ostertag1  has  pointed 
out  this  condition,  and  has  drawn  attention  to  the  nu- 
merous cases  of  intestinal  tuberculosis  in  hogs. 

116.  Bacterial  changes  in  cream.    Although  cream 
is  numerically  much  richer  in  bacteria  than  milk,  yet  the 
changes  due  to  bacterial  action  are  so  much  slower  that 
the  latter  product  usually  spoils  sooner  than  cream.     For 
this  reason,  cream  will  sour  sooner  when  it  remains  on 
the  milk  than  it  will  if  it  is  separated  as  soon  as  possible. 
This  fact  indicates  the  necessity  of  early  creaming,  so  as 
to  increase  the  keeping  quality  of  the  product,  and  is  an- 
other argument  in  favor  of  the  separator  process. 

117.  Bacteria  in  different  creaming' methods.   The 
method  used  in  creaming  has  an  important  bearing  on 
the  kind  as  well  as  the  number  of  the  bacteria  that  are 
to  be  found  in  the  cream.     The  difference  in  species  is 
largely  determined  by  the  difference  in  ripening  tempera- 
ture, while  the  varying  number  is  governed  more  by  the 
age  of  the  milk. 

1.  Primitive  gravity  methods.  In  the  old  shallow  pan 
process,  the  temperature  of  the  milk  is  relatively  high, 
as  the  milk  is  allowed  to  cool  naturally.  This  compara- 
tively high  temperature  favors  especially  the  develop- 
ment of  those  forms  whose  optimum  growing-point  is 
near  the  air  temperature.  By  this  method  the  cream 
layer  is  exposed  to  the  air  for  a  longer  time  than  any 


1  Ostertag,  Milch  Ztg.,  22:  672. 


Cream  and  Factory  By -Products.  121 

other,  and  consequently,  the  contamination  from  this 
source  is  greater.  Usually,  cream  obtained  by  the  shal- 
low pan  process  will  contain  a  larger  number  of  species 
and  also  have  a  higher  add  content. 

2.  Modern  gravity  methods.     In  the  Cooley  process,  or 
any  of  the  modern   gravity  methods  where  cold  water  or 
ice  is  used  to  lower  the  temperature,  the  conditions  do 
not  favor  the  growth  of  a  large  variety  of  species.     The 
bacterial  numbers  in  the  cream  will  depend  largely  upon 
the  way  in  which  the  milk  is  handled  previous  to  setting. 
If  milked  with  care,  and  kept  so  as  to  exclude  outside 
contamination,  the  cream  will  be  relatively  poor  in  bac- 
teria.    Only  those  forms  will  develop  in  abundance  that 
are  able  to  grow  at  the  low  temperature  at  which  the  milk 
is  set.     Cream  raised  by  this  method  is  less  frequently 
infected  with  undesirable  forms  than  that  which  is  creamed 
at  a  higher  temperature. 

3.  Centrifugal   method.     Separator   cream    should   be 
freer  from  germ-life  than  that  which  is  secured  in  any 
other  way.     It  should  contain  only  those  forms  that  have 
found  their  way  into  the  milk  during  and  subsequent  to 
the  milking,  for  the  cream  is  ordinarily  separated  so  soon 
that  there  is  but  little  opportunity  of  infection,  if  care  is 
taken  in  the  handling.     As  a  large  part  of  the  infection  of 
fresh  milk  is  due  to  the  contamination  from  the  fore  milk, 
which  usually  has  but  a  few  species,  separated  cream,  if 
handled  with  caution,  will  contain  mainly  those  species 
that  are  to  be  found  in  abundance  in  the  milk  while  it  is 
still  fresh. 

Where  milk  is  separated,  it  is  always  prudent  to  cool 
the  cream,  as  the  milk  is  generally  heated  before  separa- 
ting in  order  to  skim  efficiently. 

118.  Factory  by-products.  While  the  by-products 
in  the  manufacture  of  butter  and  cheese  are  in  a  certain 


122  Dairy  Bacteriology. 

sense  waste  products,  yet,  all  of  them  possess  sufficient 
nutrient  value  to  warrant  their  use  as  human  food  or  in 
animal  feeding.  When  handled  carelessly,  however,  their 
nutritive  value  is  much  lessened  by  the  continued  fer- 
mentations that  occur  in  them.  All  of  these  products 
are  rich  in  bacteria  owing  either  to  their  age  or  the  treat- 
ment they  have  undergone  in  the  manufacture  of  butter 
or  cheese.  It  is,  therefore,  all  the  more  essential  that 
they  should  be  kept  in  such  a  manner  as  to  check  the 
continued  development  of  germ-life  within  them. 

119.  Skim-milk.     Skimmed  milk  varies  much  in  its 
bacterial  content,  depending  upon  the  way  in  which  the 
full  milk  has  been  treated.     Milk  from  which  the  cream 
has  been  removed  by  the  shallow  setting  process  is  usu- 
ally very  rich  in  germs,  and  often  has  so  much  acid  that 
it  is  easily  recognized  by  the  taste.     Where  the  cream  is 
gathered  by  the  aid  of  ice-water,  the  temperature  is  re- 
duced to  such  an  extent  that  the  skimmed  part  is  rela- 
tively poor  in  bacteria.     Separator  skim-milk  is  treated 
in  such  a  radically  different  way  that  it  is  bacteriologic- 
ally  quite  a  different  product.     The  skimmed  part  is  sep- 
arated from  the  fat  when  the  milk  is  only  a  few  hours  old 
so  that  the  opportunity  for  germ  growth  is  relatively 
slight. 

1 20.  Buttermilk.    Buttermilk  contains  a  large  amount 
of  casein  and  sugar,  and  is,  therefore,  of  considerable 
value  for  feeding  purposes.     It  is  usually  very  rich  in 
bacteria,  sometimes  containing  even  more  than  ripened 
cream.     Pammel1  found  1,700,000  germs  per  cc.  in  but- 
termilk, while  the  organisms  in  butter  were  less  than 
half  a  million.     This  high  content  is  due  to  the  age  of 
the  milk  and  temperature  at  which  the  cream  is  previously 
ripened. 


1  Pammel,  Iowa  Expt.  Stat.,   Bull.  21. 


Cream  and  Factory  By -Products.  123 

121.  Whey.     This  by-product  of  cheese-making  also 
has  considerable  value  for  feeding  purposes.     Like  sep- 
arator  skim-milk,   it   is   drawn  at   a    temperature  that 
greatly  favors  bacterial  development.      Compared  with 
the  germ  content  of  the  raw  milk,  whey  possesses  fewer 
organisms  than  skim- milk  or  buttermilk  kept  under  sim- 
ilar conditions,  as  a  larger  part  of  the  bacteria  present  in 
the  milk  are  caught  in  the  coagulated  curd.     The  pres- 
ence of  these  in  whey,  if  left  to  cool  naturally,  soon  sours 
the  product,  as  the  milk-sugar  is  further  converted  into 
various  acids.      Both  whey  and  skim-milk  for  feeding 
purposes  should  be  carefully  handled  in  order  to  get  the 
most  out  of  them.    If  the  ferments  are  allowed  to  develop, 
the  sugar  is  changed  into  various  acids,  the  albumen  un- 
dergoes putrefactive  changes,  and  much  of  the  feeding 
value  is  lost. 

122.  Treatment  of  whey  and  skim-milk  tanks. 
The  vats  for  factory  by-products  are  often  made  of  wood; 
consequently,  they  are  difficult  to  clean  thoroughly,  even 
if  that  procedure  is  attempted.     If  not  carefully  cleansed 
and  sterilized  by  steam  each  day,  the  particles  of  milk  or 
whey  that  adhere  to  the  walls,  quickly  sour,  and  so  infect 
the  material  that  is  stored  in  the  vat  on  the  succeeding 
day.     In  this  way  the  vat  becomes  a  center  of  bacterial 
infection.     Often,  too,  this  already  contaminated  waste 
product  is  allowed  to  stand  in  cans  after  it  is  taken  back 
to  the  farms  until  it  is  thoroughly  soured.     Such  fer- 
mented food  has  only  a  minimum  value,  as  much  of  its 
nutritive  worth   is  gone.      Vats  for  these   by-products 
should  be  constructed  from  galvanized  iron  and  arranged 
to  empty  by  gravity.     The  vats  and  supply  pipes  should 
be  carefully  cleaned  each  day  as  much  as  any  other  part 
of  the  factory. 

The  trouble  arising  from  sour  whey  and  sour  milk  is 


124 


Dairy  Bacteriology. 


made  still  worse  by  the  practice  of  returning  these  highly 
contaminated  fluids  to  the  farm  in  the  same  set  of  cans 
that  are  used  for  the  transportation  of  milk.  Filthy  whey- 
vats  and  cans  are  responsible  for  much  of  the  tainted 
milk  that  is  seen  in  factories.  It  is  necessary  that  the 
cheese-maker  should  have  his  waste-vat  free  from  sour 
and  half-fermented  whey  before  he  can  charge  all  of  the 
blame  upon  the  patron  who  brings  bad  milk. 


FIG.  30.  A  Swiss  cheese  factory,  showing  careless  way  in  which  the  whey  is 
handled.  Each  patron's  share  is  placed  in  a  barrel,  from  which  it  is  removed  by 
him.  No  attempt  is  made  to  cleanse  these  receptacles. 

Fig.  30  illustrates  the  manner  in  which  the  whey  is 
distributed  and  cared  for  in  many  factories  that  make 
Swiss  cheese  particularly.  The  hot  whey  is  run  out 
through  the  trough  from  the  factory  into  a  large  trough 
that  is  placed  over  a  row  of  barrels,  as  seen  in  the  fore- 
ground. Each  patron  thus  has  allotted  to  him  his  proper 
share,  which  he  removes  day  by  day.  No  attempt  is 
made  to  clean  out  these  receptacles,  and  the  inevitable 


Cream  and  Factory  By -Products.  125 

result  is  that  they  become  a  foul,  stinking  mass  in  hot 
weather.  This  material  is  usually  carried  home  in  the 
same  set  of  cans  in  which  the  milk  is  brought  and  as  the 
cans  are  often  insufficiently  cleaned,  the  new  milk  is  in- 
fected with  undesirable  organisms.  That  such  a  custom 
should  have  grown  up  in  the  Swiss  cheese  industry,  where 
on  account  of  the  sweet  curd  nature  of  the  cheese,  all 
possible  precautions  should  be  taken  to  insure  the  best 
quality  of  milk,  illustrates  the  necessity  of  bacteriologi- 
cal principles  being  thoroughly  instilled  into  the  dairy 
industry. 

123.  Preserving- factory  by-products.   Factory  by- 
products on  account  of  their  chemical  constitution  and 
their  bacterial  flora  soon  decompose,  and  a  considerable 
proportion  of  the  nutrients  are  lost.     This  has  led  to  the 
introduction  in  some  factories  of  measures  that  tend  to 
prolong  the  keeping  quality  of  the  whey  and  skim-milk, 
although  the  value  of  these  products  do  not  warrant  much 
expense.      The  conditions  under  which  skim-milk  and 
whey  are  produced  facilitate  the  rapid  development  of 
bacteria.     The  continued  fermentation  of  these  materials 
may  be  inhibited  in  part  as  follows : 

1.  Quick  cooling  to   a   temperature   unfavorable   for 
germ  development. 

2.  Pasteurizing  or  scalding. 

Where  fed  immediately,  it  is  hardly  worth  while  to 
treat  these  by-products,  as  they  will  remain  sweet  for 
several  hours.  In  some  factories  the  skim-milk  is  heated 
by  introducing  live  steam.  Where  milk  is  pasteurized 
for  butter- making,  the  keeping  quality  of  the  skim-milk 
is  increased  24-48  hours  thereby. 

124.  Skim-milk  a  distributor  of  disease.   In  some 
countries,  notably  in  Denmark  and  Germany,  tubercu- 
losis  is  so  common  among  cattle  that  skim-milk  from 


126  Dairy  Bacteriology. 

factories  becomes  a  serious  menace  when  fed  to  young 
stock.  To  prevent  distribution  in  this  manner,  compul- 
sory legislation  requires  that  the  skim-milk  shall  be 
pasteurized  at  a  temperature  of  176°  F.,  in  order  to 
destroy  the  bacilli.  Storch  has  recently  devised  a  test 
that  can  be  easily  applied  to  milk  to  determine  whether 
such  treatment  has  been  carried  out.  It  rests  upon  the 
following  principle:  Milk  contains  a, certain  substance, 
(presumably  an  enzyme,1)  that  decomposes  hydrogen 
peroxid.  If  milk  is  heated  to  80°  C.  (176°  F.,)  or  above, 
this  reaction  ceases.  When  potassium  iodid  and  starch 
are  added  to  unheated  milk  and  the  same  treated  with  di- 
lute hydrogen  peroxid,  the  fluid  assumes  a  blue  color  due 
to  the  action  of  released  iodin  upon  the  starch. 

A  striking  illustration  of  the  danger  that  may  come 
from  a  skim-milk-supply  that  is  infected  with  disease  bac- 
teria is  seen  in  the  Welply  epidemic  of  typhoid  fever  in 
England  in  1893,  where  twenty- three  cases  of  this  disease 
developed  in  the  families  of  patrons  of  a  single  factory, 
the  milk- supply  of  which  became  infected  and  was  spread 
by  means  of  the  skim -milk. 

Foot  and  mouth  disease  of  cattle  is  often  disseminated 
in  the  same  way.  During  the  last  decade,  this  disease 
has  been  very  severe  in  certain  parts  of  Europe.  The 
virus  of  the  disease  can,  however,  be  destroyed  by  the 
use  of  heat.  The  regulation  of  the  Prussian  govern- 
ment require  milk  of  all  diseased  animals  to  be  heated  to 
212°  F.  or  to  194°  F.  for  fifteen  minutes. 

1  Storch  believes  it  to  be  the  unorganized  ferment  discovered  in  milk 
by  Babcock  and  Russell,  14th  Wis.  Expt.  Stat.,  p.  77,  1897. 


CHAPTER  IX. 
BACTERIA  IN  BUTTER-MAKING. 

125.  Sweet  and  ripened  cream  butter.  The  pecu- 
liar qualities  of  ordinary  butter  are  so  dependent  upon 
the  relation  of  bacteria  to  cream,  that  in  order  to  under- 
stand the  subject  aright,  it  is  necessary  to  consider,  first, 
their  effect  in  cream.  If  butter  is  made  from  fresh  sweet 
cream,  it  has  a  delicate,  although  unpronounced  flavor 
that  differs  .considerably  from  the  usual  product.  With 
the  great  bulk  of  the  commercial  product,  the  cream  is 
allowed  to  stand  for  a  certain  length  of  time,  during 
which  it  undergoes  a  series  of  fermentations  technically 
known  as  "ripening." 

American  consumers  have  become  accustomed  to  the 
peculiar  properties  of  acid  cream  butter,  and  as  yet,  there 
is  but  little  demand  for  the  sweet  cream  product.  The 
keeping  quality  of  the  latter  is  relatively  poor  compared 
with  that  made  from  ripened  cream,  so  that  it  is  only 
adapted  for  immediate  consumption. 

The  germ  content  and  the  quality  of  ripened  cream 
butter  varies  materially,  depending  upon  the  way  the 
cream  is  handled. 

In  dairy  butter-making,  the  cream  is  usually  gathered 
by  the  gravity  process,  and  is  ripened  in  various  ways. 
In  creamery  butter  two  methods  of  securing  cream  are  in 
vogue.  The  more  primitive  way  is  where  the  cream  is 
separated  by  gravity  and  is  taken  to  the  factory  when  it 
is  nearly  ready  for  churning.  The  more  modern  method 
is  to  bring  the  whole  milk  to  the  creamery  where  the 

cream  is  removed  by  centrifugal  separation.     In  the  one 

[127] 


128  Dairy  Bacteriology. 

case,  the  fermentative  changes  are  well  under  way  when 
it  reaclies  the  central  station,  and  as  the  cream  is  secured 
from  a  number  of  different  persons  in  a  partially  ripened 
condition,  it  is  far  from  being  uniform  in  character. 
Where  centrifugal  cream  separators  are  used,  the  cream 
is  secured  sweet,  thereby  permitting  a  supervision  of  the 
ripening  process  at  the  central  station  where  the  butter 
is  made. 

This  control  of  the  ripening  process  exerts  a  profound 
influence  on  the  character  of  the  fermentation  that  takes 
place,  and  naturally  modifies  the  product.  The  uni- 
formity of  methods  employed  necessarily  has  a  tendency 
to  render  the  product  more  uniform. 

A.     BENEFICENT    ACTION    OF    BACTERIA    IN    BUTTER. 

126.  Ripening-  of  cream.  The  ripening  changes  that 
occur  in  cream  are  exceedingly  complex,  and  our  knowl- 
edge concerning  the  same  is  as  yet  far  from  satisfactory. 
The  present  belief  is  that  these  fermentations  are  largely 
the  result  of  bacterial  action,  as  germ  growth  is  very 
abundant.  Conn1  found  in  fresh  cream  4,060,000  bac- 
teria, while  the  ripened  cream  contained  346,000,000. 

In  the  ripening  of  cream  at  least  three  different  fac- 
tors are  to  be  taken  into  consideration,  the  development 
of  acid,  flavor  and  aroma.  Much  confusion  in  the  past 
has  arisen  from  a  failure  to  discriminate  between  these 
factors.  While  all  three  of  these  qualities  are  produced 
simultaneously  in  ordinary  ripening,  it  does  not  neces- 
sarily follow  that  any  single  organism  is  able  to  develop 
all  three  qualities.  If  this  ripening  is  allowed  to  go  too 
far,  undesirable  rather  than  beneficial  decomposition 
products  are  produced.  These  greatly  impair  the  value 
of  butter,  so  that  in  a  sense  the  commercial  value  of  this 

1  Conn,  Storr's  Agric.  Expt.  Stat.,  7:  72,  1894. 


Bacteria  in  Butter- Making .  129 

product  is  dependent  upon  the  character  and  the  extent 
of  the  ripening  changes.  A  careful  study  of  large  num- 
bers of  dairy  bacteria  as  has  been  done  by  Conn,  shows 
that  these  problems  can  be  separated,  if  pure  cultures 
are  used. 

127.  Development  of  acid.    In  the  ripening  of  cream 
acid  is  almost  invariably  developed.     This  development 
is  essential,  as  it  renders  churning  easier,  and  increases 
the  yield  of  butter.     The  acid  formed  is  largely  lactic, 
and  is  produced  by  a  decomposition  of  the  milk-sugar. 
So  characteristic  of  cream-ripening  is  this  production  of 
acid  that  the  process  is  frequently  spoken  of  as  souring, 
although  it  must  be  kept  in  mind  that  the  process  in- 
volves much  more  than  mere  acid  production.     Normal 
butter  made  from  ripened  cream  always  has  a  character- 
istic flavor,  which  quality  is  distinct  from  the  acid  for- 
mation.    This  difference  is  seen  in  the  results  obtained 
by  Tiemann1  when  cream  is  ripened  by  the  addition  of 
hydrochloric  acid.     When  so  treated,  cream  can  be  easily 
churned,  but  the  product  is  lacking  in  the  aromatic  qual- 
ities found  in  normally  ripened  cream. 

128.  Flavor.     The  flavor  of  butter  is  that  quality  that 
is  judged  by  the  sense  of  taste.     Good,  bad,  or  indiffer- 
ent flavors  may  be  produced  in  butter  as  a  result  of  bac- 
terial action.     In  fact,  the  production  of  flavor  in  ripened 
cream  butter  is  in  large  part  due  to  the  fermentative  pro- 
ducts of  microbes,  although  it  is  to  some  extent  modified 
by  the  inherent  qualities  of  the  milk  that  are  induced  by 
the  character   of   the  feed   which  is  consumed  by  the 
animal. 

129.  Aroma.     The  aroma  of  butter  is  often  confounded 
with  that  of  flavor,  but  this  quality  is  dependent  upon 

1  Tiemann,  Milch  Ztg.,  23:  701. 
9-B. 


130  Dairy  Bacteriology. 

the  presence  of  volatile  decomposition  products  that 
appeal  to  the  sense  of  smell  rather  than  taste.  As  a  rule 
butter  is  judged  by  this  characteristic,  a  good  flavor  ac- 
companying a  desirable  aroma,  but  while  these  two 
qualities  are  frequently  present  in  the  same  butter  made 
from  naturally  ripened  cream,  it  does  not  by  any  means 
follow  that  one  is  directly  dependent  upon  the  other. 

130.  Origin  of  flavor  and  aroma.  The  source  from 
which  these  delicate  and  evanescent  qualities  are  derived 
is  not  yet  definitely  known.  Two  opposing  views  have 
been  advanced.  Storch1  holds  that  the  flavors  are  pro- 
duced from  the  decomposition  of  milk-sugar  and  the  ab- 
sorption of  the  volatile  flavors  by  the  butter  fat.  Conn2 
believes  that  the  nitrogenous  elements  in  the  cream  func- 
tion as  food  materials  from  which  are  formed  various 
decomposition  products,  among  which  is  the  desired  aro- 
matic substance.  The  change  is  unquestionably  a  com- 
plex one,  and  cannot  be  explained  as  a  single  fermentation. 

There  is  no  longer  much  doubt  but  that  both  acid-form- 
ing and  casein- digesting  species  are  concerned  in  the  pro- 
duction of  proper  flavors  as  well  as  aromas.  The  re- 
searches of  Conn3,  who  has  studied  this  question  most 
exhaustively,  indicate  that  both  of  these  types  of  decom- 
position participate  in  the  production  of  flavor  and  aroma. 
He  has  shown  that  both  flavor  and  aroma  production  are 
independent  of  acid,  that  many  good  flavor- producing 
forms  belong  to  that  class  which  renders  milk  alkaline, 
or  does  not  change  the  reaction  at  all.  Some  of  these 
species  liquefied  gelatin  and  would  therefore  belong  to 
the  casein- dissolving  class.  Those  species  that  produced 
bad  flavors  are  also  included  in  both  fermentative  types. 

1  Storch,  Nogle.  Unders.  over  Floed.  Syrning,  1890. 

2  Conn,  6th  Storrs  Expt.  Stat.,  p.  66,  1893. 

3  Conn,  9th  Storrs  Expt.  Stat.,  p.  17,  1896. 


Bacteria  in  Butter-Making.  131 

Conn  has  found  a  number  of  the  organisms  that  are 
favorable  flavor- producers ;  in  fact  they  were  much  more 
numerous  than  desirable  aroma-yielding  species.  None 
of  the  favorable  aroma  forms  were  lactic  acid  species l . 

131.  Effect  of  bacteria  on  ripening".     The  majority 
of  bacteria  in  ripening  cream  do  not  seem  to  exert  any 
particular  influence  in  butter.     A  considerable  number 
are  positively  beneficial,  inasmuch  as  they  produce  a  good 
flavor  and  aroma,  so  far  as  these  qualities  are  pronounced. 
A  more  limited  number  are  concerned  in  the  production 
of  undesirable  ripening  changes2.     A  knowledge  of  the 
conditions  that  control  the  development  of  these  respect- 
ive types  of  ripening  is  of  greatest  practical  value  to  the 
butter-maker.     If  it  were  possible  to  have  these  condi- 
tions under  exact  control,  then  the  butter  industry  would 
be  reduced  to  the  terms  of  an  exact  science. 

132.  Methods  of  cream-ripening".     1.  Natural  ri- 
pening.    The  simplest  and  oldest  method  is  where  the 
cream  is  allowed  to  ripen  without  any  special  control,  the 
only  artificial  aid  being  a  regulation  in  a  crude  way  of 
the  temperature.     The  whole  process  is  a  let-alone  one. 
The  ripened  product  varies  much  in  degree  of  ripeness ; 
also  in  the  kind  of  fermentation,  depending  upon  the 
various  species  of  bacteria  that  have  happened  to  gain 
access  to  the  milk. 

The  results  obtained  by  these  cruder  methods  are 
usually  fair,  but  simply  because  the  majority  of  the  or- 
ganisms normally  present  in  ordinarily  clean  milk  are 
such  as  are  unable  to  produce  marked  flavors  of  an  un- 
desirable character. 

2.  Natural  starters.  In  the  above  method  the  rate  of 
ripening  is  often  irregular,  and  to  overcome  this,  starters 

1  Weigmann  has  reached  these  same  results  (Milch  Ztg.,  p.  793, 
1891). 

2Eckles.  Cent.  f.  Bakt.,  II.  Abt.,  4:  730.  1898. 


132  Dairy  Bacteriology. 

have  been  introduced  as  a  means  of  hastening  and  rendering 
the  ripening  process  more  nniform.  In  the  use  of  these, 
bacterial  growths  are  added  to  assist  in  the  ripening  of 
cream,  although  generally  no  knowledge  of  the  bac- 
terial processes  involved  has  until  recently  been  taken 
into  consideration.  These  methods  are  a  great  improve- 
ment over  the  previous  let-alone  policy  of  cream-ripening 
as  one  is  often  able  to  exclude  in  this  way,  undesirable 
fermentative  changes.  For  starters  of  this  sort,  several 
different  materials  are  used,  as  sour-milk,  buttermilk,  or 
whey,  all  of  which  are  liquids  rich  in  bacteria. 

3.  Artificial  starters  (bacterial  cultures}.  A  much 
more  scientific  method  of  ripening  cream  has  been  intro- 
duced within  the  last  few  years  and  is  now  being  exten- 
sively used  in  Scandinavian,  Danish,  and  German  dairies. 
This  method  dates  from  the  labors  of  Storch,1  who  in 
1890  proposed  and  introduced  the  use  of  pure  cultures 
that  have  been  selected  on  account  of  their  ability  to 
produce  a  desirable  ripening  change  in  cream.  4His  in- 
vestigations opened  up  a  new  field  of  research  which  has 
since  been  diligently  prosecuted  in  both  Europe  and 
America. 

133.  Principles  of  pure  culture  cream-ripening'. 
This  method  rests  on  the  principle  that  a  selected  culture 
that  has  been  chosen  on  account  of  its  favorable  ripen- 
ing qualities  will  be  able  to  produce  a  similar  fermenta- 
tion in  cream,  if  the  proper  conditions  are  present.  To 
get  the  best  results,  it  is  evident  that  the  selected  culture 
will  have  favorable  conditions  for  its  development,  if  the 
pre-existing  bacteria  in  the  cream  are  first  destroyed.  In 
this  way  competition  is  reduced  to  a  minimum.  This 
preparation  of  the  cream  is  accomplished  by  pasteurizing 

1  Storch,  Milch  Zeit.,  1890,  p.  304. 


Bacteria  in  Butter-Making.  133 

the  same.  This  destroys  for  the  most  part  the  vegeta- 
tive bacteria,  leaving  the  latent  spore  forms  in  the  cream. 
The  addition  then  of  a  properly  propagated  starter  gives 
the  selected  organism  snch  an  impetus  that  it  should  be 
able  to  overcome  any  other  organism  in  the  cream.  Where 
pure  cultures  are  employed  extensively,  as  in  Denmark 
and  Germany,  this  method  of  cream-ripening  is  followed. 

The  attempt  has  been  made  to  use  these  culture  start- 
ers in  raw  sweet  cream,  but  it  can  scarcely  be  expected 
that  the  most  beneficial  results  will  be  attained  in  this 
way.  This  method  has  been  justified  on  the  basis  of  the 
following  experiments.  Where  cream  is  pasteurized  and 
no  starter  is  added,  the  spore-bearing  forms  frequently 
produce  undesirable  flavors.  These  can  almost  always 
be  controlled  if  a  culture  starter  is  added,  the  obnoxious 
form  being  repressed  by  the  presence  of  the  added  starter. 
This  condition  is  interpreted  as  indicating  that  the  addi- 
tion of  a  starter  to  cream  which  already  contains  devel- 
oping bacteria  will  prevent  those  originally  present  in  the 
cream  from  growing.1  This  repressive  action  of  one 
species  on  another  is  a  well-known  bacteriological  fact, 
but  it  must  be  remembered  that  such  an  explanation  is 
only  applicable  in  those  cases  where  the  culture  organ- 
ism exercises  a  direct  prejudicial  influence  on  the  exist- 
ing flora  of  the  cream. 

If  the  culture  organism  is  added  to  raw  milk  or  cream 
which  already  contains  a  flora  that  is  eminently  adapted 
for  development  in  this  medium,  it  is  quite  doubtful 
whether  the  culture  organism  would  gain  the  supremacy 
in  the  ripening  cream.  The  above  method  of  adding  a 
culture  to  raw  cream  renders  cream-ripening  details  less 
burdensome,  but  at  the  same  time  Danish  experience, 


1  Conn,  9th  Storrs  Expt.  Stat.,  p.  25,  1896. 


134  Dairy  Bacteriology. 

which  is  entitled  to  most  credence  on  this  question1,  is 
opposed  to  this  method. 

1 34.  Pasteurizing*  milk  or  cream  for  butter.  To 
pasteurize  for  butter-making,  it  is  not  necessary  to 
carry  out  the  process  in  the  same  stringent  manner  as 
when  it  is  done  to  preserve  milk  or  to  free  it  from  pos- 
sible disease-breeding  bacteria.  A  temperature  of  140° 
F.  for  ten  minutes  is  fatal  to  most  of  the  lactic  acid 
group,  but  in  pasteurizing  for  butter-making,  it  is  cus- 
tomary to  heat  the  milk  to  a  higher  temperature  (155- 
175°  F.).  This  is  done  for  two  purposes.  First,  the 
milk  is  generally  heated  in  a  continuous-flow  machine; 
consequently,  the  maximum  temperature  is  not  main- 
tained for  more  than  part  of  the  time  required  to  go 
through  the  machine.  Second,  the  wide- spread  preva- 
lence of  tuberculosis,  and  foot  and  mouth  disease  in  some 
of  the  dairy  regions  of  Europe  makes  it  necessary  to  heat 
the  skim-milk  high  enough  to  destroy  the  seeds  of  these 
diseases. 

In  pasteurizing  for  butter-making,  it  is  also  possible 
to  drive  off  many  taints  that  have  been  absorbed  directly 
from  the  cow  or  through  exposure  to  foul  odors2.  Some 
states  of  the  Atlantic  sea-board  are  so  infested  with  wild 
garlic  that  the  milk-supply  is  rendered  practically  worth- 
less for  dairy  purposes.  If  this  is  treated  by  pasteur- 
ization, it  is  often  possible  to  eliminate  in  part  such 
taints,  so  that  a  fairly  good  product  of  butter  can  be 
made. 

In  pasteurizing  for  butter-making,  two  methods  are  in 
vogue  in  Denmark  where  the  greatest  activity  exists  in 
regard  to  this  matter.  There,  either  the  whole  milk  is 

1  99$  of  the  Danish  creameries  have  pasteurizers,  and  over  90$  pas- 
teurize the  cream. 

2  McKay,  la.  Expt.  Stat.,  Bull.  32,  p.  477. 


Bacteria  in  Butter- Making. 


135 


handled  or  the  cream  is  pasteurized  after  separation. 
The  necessity  of  heating  the  skim-milk  to  destroy  the 
seeds  of  tubercle  in  the  milk- supply  makes  it  advisable 
to  heat  the  whole  milk,  thus  subserving  two  purposes  by 
the  same  operation. 

135.  Pasteurizing"  apparatus  for  butter- making*. 
Most  of  the  European  pasteurizing  machinery  has  been 
devised  from  the  butter-maker's  standpoint.  They  are, 
therefore,  machines  that  have  a  large  capacity,  as  this 
factor  is  generally  regarded  as  of  chief  importance  in 
pasteurizing  for  butter- making.  To  fulfill  this  condition, 
most  of  them  are  arranged  with  a  continuous  inflow  and 
outflow.  Such  an  arrangement  militates  considerably 

against  the  efficiency  of  any 
machine  as  a  bacteria- de- 
stroying agent.  In  a  test 
made  by  the  writer1  of 
Reid's  pasteurizer  (fig. 31), 
which  is  a  modification  of  a 
Danish  machine,  it  was 
found  that  the  bacterial 
content  of  the  pasteurized 
milk  varied  greatly,  rang- 
ing from  50-99%,  although 
on  the  whole  the  germicidal 
effect  was  very  marked. 

136.  Attributes  desir- 
able in  pure  culture 
starters.  In  adding  a  pure 

Reid's  pasteurizer  for  butter-  culture  starter  to  cream,  it 
is  desirable,  if  possible,  to 
use  a  ferment  that  possesses  the  following  character- 
istics : 


FIG.  31. 
making. 


1  Wis.  Expt.  Stat.,  Bull.  69,  p.  11. 


136  Dairy  Bacteriology. 

1.  Vigorous  growth  in  milk  at  ordinary  ripening  tem- 
peratures. 

2 .  Ability  to  form  acid  so  as  to  facilitate  churning  and 
increase  the  yield  of  butter. 

3.  Able  to  produce  a  clean  flavor  and  desirable  aroma. 

4.  Impart  a  good  keeping  quality  to  butter. 

5.  Not  easily  modified  in  its  flavor- producing  qualities 
by  artificial  cultivation. 

These  different  conditions  are  difficult  to  attain,  for  the 
reason  that  some  of  them  seem  to  be  in  part  incompat- 
able.  Weigmann1  found  that  a  good  aroma  was  gener- 
ally an  evanescent  property,  and  therefore,  opposed  to 
good  keeping  quality.  Conn  has  shown  that  the  functions 
of  acid-formation,  flavor  and  aroma  production  are  not 
necessarily  related,  and  therefore,  the  chances  of  finding 
a  single  organism  that  possesses  all  the  desirable  at- 
tributes are  not  very  good.  It  may  well  be  asked  if  the 
conditions  are  so  peculiar,  how  is  it  that  a  good  product 
is  ever  made  where  no  special  attention  is  given  this 
matter  of  culture  starters?  The  reason  for  this  is  that 
the  ripening  of  cream  is  not  a  single  type  of  fermentation, 
but  a  complex  process  in  which  various  organisms  may 
participate. 

In  all  probability  no  one  germ  possesses  all  of  the  de- 
sirable qualities,  but  the  resultant  of  the  action  of  several 
forms  may  produce  a  good  product2.  This  idea  has  led 
to  the  attempt  of  mixing  selected  organisms  that  have 
been  chosen  on  account  of  certain  favorable  characteris- 
tics which  they  might  possess.  The  difficulty  of  main- 
taining such  a  composite  culture  in  its  correct  proportions 
when  it  is  being  propagated  in  the  creamery  is  seemingly 
well  nigh  insuperable,  as  one  organism  is  very  apt  to 
develop  more  or  less  rapidly  than  the  other. 

1  Weigmann,  Landw.  Woch.  f.  Schl.  Hoi.,  No.  2,  1890. 

2  Weigmann,  Cent.  f.  Bakt.,  II.  Abt.,  3:  497,  1897. 


Bacteria  in  Butter- Making.  137 

137.  Reputed    advantages    of    culture    starters. 

1.  Flavor  and  aroma.  These  are  factors  of  greatest  im- 
portance, as  the  market  value  is  determined  largely  by 
this  quality.  Ferments  of  this  sort  rarely  produce  any 
higher  or  more  pronounced  flavor  or  aroma  than  the  best 
of  the  natural  product,  but  they  possess  the  reputation  of 
producing  a  clean,  mild  flavor  that  is  very  desirable  when 
the  market  becomes  accustomed  to  it. 

2.  Uniformity  of  product.     Culture  starters  produce  a 
more  uniform  product,  because  the  type  of  fermentation 
does  not  vary  from  day  to  day.     Even  the  best  of  butter- 
makers  sometimes  fail  to  make  a  standard  product,  owing 
to  their  inability  to  fully  control  the  character  of  the 
ripening  process. 

3.  Keeping  quality  of  product.     Butter  made  from  pas- 
teurized cream  to  which  a  good,  pure  starter  has  been 
added  will  keep  much  better  than  the  ordinary  product, 
because  the  diversity  of  the  flora  is  less  and  the  milk  is 
not  likely  to  contain  those  organisms  that  produce  an 
"  off  "  condition. 

4.  Elimination  of  butter  defects.     One  of  the  greatest 
advantages  is  its  application  to  factories  that  are  unable 
to  make  good  butter  on  account  of  some  hidden  trouble 
due  to  bacteria. 

138.  Pure  cultures  vs.  home-made  starters.    The 
question  as  to  the  relative  merits  of  pure  or  domestic  fer- 
ments as  starters  should  be  answered  in  the  light  of  vary- 
ing conditions.     In  the  present  development  of  the  pure 
culture  system,  no  culture  on  the  market  possesses  such 
superior   advantages  over  a  first-class  domestic  starter 
especially  as  to  flavor  and  aroma,  that  it  can  be  unquali- 
fiedly recommended  as  better.     The  main  advantage  of 
the  culture  system  lies  in  the  uniformity  of  the  product. 
The  elimination  of  the  risk  that  always  attends  in  a  greater 


138  Dairy  Bacteriology. 

or  smaller  degree  the  use  of  an  ordinary  starter,  is  of 
material  worth.  If  great  care,  however,  is  taken  in  the 
selection  of  the  domestic  starter,  and  it  is  propagated  un- 
der as  equally  favorable  conditions  for  the  retention  of 
its  original  purity,  as  can  easily  be  done  with  the  com- 
mercial- starter,  then  there  is  no  reason  why  practically  as 
good  results  cannot  be  obtained  in  the  use  of  one  as  the 
other. 

139.  Imperfections    in    system.     The   commercial 
value  of  butter  is  so  dependent  upon  the  character  of  the 
flavor  that  the  effect  of  a  starter  on  this  factor  will  always 
be  of  supreme  importance .     At  the  present  time  no  pure 
culture  starter  is  known  that  is  capable  of  imparting  a 
much  higher  flavor  than  is  obtained  in  the  regular  way l . 
Consequently,  the  maker  is  unable  to  secure  a  higher 
price  for  his  butter  than  in  the  better  creameries  where 
the  older  method  is  used. 

Another  decided  disadvantage  that  occurs  where  the 
cream  is  pasteurized  in  conjunction  with  the  use  of  a 
pure  culture,  is  the  effect  on  the  grain  or  body  of  the 
product.  As  judged  by  our  present  American  standards, 
the  grain  of  the  butter  is  materially  lowered  where  the 
cream  has  been  heated. 2  This  undoubtedly  would  not  be 
so  considered  in  the  foreign,  especially  the  English  or 
Danish  markets,  but  so  long  as  our  market  standards 
exact  the  qualities  now  demanded  on  grain,  this  will  to 
some  extent  handicap  pasteurized  butter. 

140.  Propagation  of  starter  for  cream-ripening-. 
The  preparation  and  propagation  of  a  starter  for  cream- 
ripening  is  a  process  involving  considerable  bacteriologi- 
cal knowledge,  whether  the  starter  is  of  domestic  origin 
or  prepared  from  a  pure- culture  ferment.     In  any  event, 

1  This  statement  is  confined  to  American  conditions. 
2Farrington  and  Russell,  Wis.  Expt.  Stat.,  Bull.  69,  p.  28. 


Bacteria  in  Butter- Making. 


139 


it  is  necessary  that  the  starter  should  be  handled  in  a 
way  so  as  to  prevent  the  introduction  of  foreign  bacteria 
as  far  as  this  is  possible.  The  following  points  should 
be  kept  in  mind  in  manipulating  the  starter: 


FIG.  32.    Apparatus  for  sterilizing  and  propagating  the  starter 

1.  If  a  pure-culture  ferment  is  used,  see  that  it  is  fresh 
and  that  the  seal  has  not  been  disturbed. 

2.  If  a  home-made  starter  is  employed,  use  the  great- 


140  Dairy  Bacteriology. 

est  possible  care  in  selecting  the  milk  that  is  to  be  used 
as  a  basis  for  the  starter. 

3.  For  the  propagation  and  perpetuation  of  the  starter 
from  day  to  day,  it  is  necessary  that  the  same  should  be 
grown  in  milk  that  is  as  germ-free  as  it  is  possible  to 
secure  it.     For  this  purpose  sterilize  some  fresh  skim- 
milk  in  a  covered  can  that   has  previously  been  well 
steamed.    This  can  be  done  easily  by  setting  cans  contain- 
ing skim-milk  in  a  vat  filled  with  water  (fig.  32  A)  and 
heating  the  same  to  approximately  the  boiling  point. 
The  temperature  should  be  maintained  for  an  hour  or 
more.     This  destroys  all  but  the  most  resistant  spore- 
bearing  organisms 1 . 

4.  After  the  heated  milk  is  cooled  down  to  about  70° 
F.,  it  can  be  inoculated  with  desired  culture.     Some- 
times it  is  desirable  to  ' '  build  up  ' '  the  starter  by  prop- 
agating it  first  in  a  smaller  volume  of  milk,  and  then 
after  this  has  developed,  adding  it  to  a  larger  amount. 

This  method  is  of  particular  value  where  a  large  amount 
of  starter  is  needed  for  the  cream -ripening. 

5.  After  the  milk  has  been  inoculated,  it  should  be  kept 
at  a  temperature  that  is  suitable  for  the  rapid  devel- 
opment of  the  contained  bacteria,  60-70°  F.,  which  tem- 
perature should  be  kept  as  constant  as  possible. 

6.  This  can  best  be  done  by  filling  water- vat  (A)  with 
water  and  heating  the  same  to  desired  temperature,  cov- 
ering the  vat  with  a  wooden  cover  or  heavy  cloth  during 
the  night  to  maintain  proper  temperature. 

7.  The  starter  should  not  be  thoroughly  curdled  and 
solid  when  it  is  needed  for  use,  but  should  be  well  soured 

1  A  number  of  tests  made  by  the  writer  at  the  Wis.  Dairy  School 
Creamery,  showed  that  the  bacteria  in  skim-milk  were  practically 
destroyed,  only  7-30  bacteria  per  cc.  remaining  after  this  sterilizing 
process. 


Bacteria  in  Butter-Making.  141 

and  partially  curdled.     This  point  is  of  importance  for 
the  following  reasons : 

a.  It  is  difficult  to  thoroughly  break  up  curd  particles 
if  the  starter  is  completely  curdled.     If  these  curd  masses 
are  added  to  ripening  cream,  white  specks  may  appear  in 
the  butter. 

b.  The  vigor  of  the  starter  is  undoubtedly  stronger 
when  the  milk  is  on  the  point  of  curdling  than  it  is  after 
the  curd  has  been  formed  some  time.     The  continued  for- 
mation of  lactic  acid  kills  many  of  the  bacteria  and  thus 
weakens  the  fermentative  action. 

8.  The  starter  should  be  propagated  from  day  to  day  by 
adding  a  small  quantity  to  a  new  lot  of  milk.     For  this 
purpose  two  propagating  cans  should  be  provided  (fig. 
32  B)  so  that  one  starter  may  be  in  use  while  the  other 
is  being  prepared. 

9.  After  the  starter  has  been  used  for  a  few  days,  the 
same  should  be  emptied,  and  the  can  cleaned  and  steril- 
ized by  steam  before  being  used  again. 

141.  How  long1  should  a  starter  be  propagated? 
No  hard  and  fast  rule  can  be  given  for  this,  for  it  de- 
pends largely  upon  how  carefully  the  starter  is  handled 
during  its  propagation.  If  the  starter  is  grown  in  steril- 
ized milk  kept  in  steamed  vessels  and  is  handled  with 
sterile  dippers,  it  is  possible  to  maintain  it  in  a  state  of 
relative  purity  for  a  considerable  period  of  time ;  if,  how- 
ever, no  especial  care  is  given  it,  it  will  soon  become  in- 
fected from  the  air  and  the  retention  of  its  purity  will 
depend  more  upon  the  ability  of  the  contained  organism 
to  choke  out  foreign  growths  than  upon  any  other  factor. 
While  it  is  possible  by  bacteriological  methods  to  deter- 
mine with  accuracy  the  actual  condition  of  a  starter  as  to 
its  germ  content,  still  such  methods  are  inapplicable  in 
creamery  practice.  Here  the  maker  must  rely  largely 


142  Dairy  Bacteriology. 

upon  the  general  appearance  of  the  starter  as  determined 
by  taste  and  smell.  Where  it  is  possible  to  propagate 
the  starter  as  has  been  described,  it  is  hardly  worth  run- 
ning any  risk  in  using  an  old  starter  when  a  fresh  one 
-can  be  so  easily  kept  in  condition. 

142.  Bacteria  in  butter.     As  ripened  cream  is  neces- 
sarily rich  in  bacteria,  it  follows  that  butter  will  also 
contain  germ  life  in  varying  amounts,  but  as  butter-fat 
is  not  well  adapted  for  bacterial  food,  the  number  of 
germs  in  butter  is  usually  less  than  in  ripened  cream. 

Sweet  cream  butter  is  naturally  poorer  in  germ  life 
than  that  made  from  ripened  cream.  Grotenfelt1  reports 
in  sweet  cream  butter,  the  so-called  "  Paris  butter  "  only 
120-300  bacteria  per  cc.,  while  in  butter  from  sour  cream 
2,000-55,000  germs  per  cc.  were  found.  Pammel2  found 
from  125,000-730,000  per  gram,  while  Lafar3  found  in 
butter  sold  in  Munich  from  10-20,000,000  organisms  per 
gram. 

The  germ  content  of  butter  on  the  outside  of  a  pack- 
age is  much  greater  than  it  is  in  the  middle  of  a  mass ; 
this  doubtless  being  due  to  the  freer  access  of  air  favor- 
ing the  growth  of  aerobic  forms. 

143.  Changes  in  germ  content.     The  bacteria  that 
are  incorporated  with  the  butter  as  it  first  ' '  comes ' 7  un- 
dergo a  slight  increase  for  the  first  few  days.     The  dura- 
tion of  this  period  of  increase  is  dependent  largely  upon 
the  condition  of  the  butter.     If  the  buttermilk  is  well 
worked  out  of  the  butter,  the  increase  is  slight  and  lasts 
for  a  few  days  only,  while  the  presence  of  so  nutritious  a 
medium    as   buttermilk    affords    conditions   much  more 
favorable  for  the  continued  growth  of  the  organisms. 

1  Grotenfelt-Woll,  Prin.  Mod.  Dairy  Practice,  p.  244. 

2  Pammel,  Bull.  21,  Iowa  Expt.  Stat.,  p.  801. 

3  Lafar,  Arch.  f.  Hyg.,  13:  1,  1891. 


Bacteria  in  Butter- Making.  143 

While  there  may  be  many  varieties  in  butter  when  it  is 
fresh,  they  are  very  soon  reduced  in  kind  as  well  as  num- 
ber. The  lactic  acid  group  of  organisms  disappear  quite 
rapidly ;  the  spore-bearing  species  remaining  for  a  some- 
what longer  time.  Butter  examined  after  it  is  several 
months  old  is  often  found  to  be  almost  free  from  germs. 
This  fact  is  important,  for  in  considering  the  after 
-changes  in  ' '  storage  ' '  butter,  the  relation  of  these 
changes  to  the  germ  life  would  naturally  be  considered. 

In  the  manufacture  of  butter  there  is  much  that  is  de- 
pendent upon  the  mechanical  processes  of  churning, 
washing,  salting,  and  working  the  product.  These  pro- 
cesses do  not  involve  any  bacteriological  principles  other 
than  those  that  are  incident  to  cleanliness.  The  cream, 
if  ripened  properly,  will  contain  such  enormous  numbers 
of  favorable  forms  that  the  access  of  the  few  organisms 
that  are  derived  from  the  churn,  the  air,  or  the. water  in 
washing  will  have  little  effect,  unless  the  conditions  are 
abnormal. 

144.  Rancid  change  in  butter.  Fresh  butter  has  a 
peculiar  aroma  that  is  very  desirable  and  one  that  enhances 
the  market  price,  if  it  can  be  retained;  but  this  delicate 
flavor  is  more  or  less  evanescent,  soon  disappearing,  even 
in  the  best  makes.  While  a  good  butter  loses  with  age 
some  of  the  peculiar  aroma  that  it  possesses  when  first 
made,  yet  a  gilt-edged  product  should  retain  its  good 
keeping  qualities  for  some  length  of  time.  All  butters, 
however,  sooner  or  later  undergo  a  change  that  render 
them  worthless  for  table  use.  This  change  is  usually  a 
rancidity  that  is  observed  in  all  stale  products  of  this 
•class.  The  cause  of  this  rancid  condition  in  butter  has 
been  attributed  to  the  action  of  living  organisms,  partic- 
ularly those  that  form  butyric  acid,  to  the  influence  of 
light,  of  air,  etc. 


144  Dairy  Bacteriology. 

In  rancid  butter,  butyric  and  allied  acids  are  always 
found,  and  it  was  supposed  for  a  long  time  that  the 
change  was  a  butyric  fermentation  inaugurated  by  some 
of  the  butyric  organisms  that  are  found  so  commonly 
in  milk  and  cream. 

Ritsert1  found  that  sterile  butter  became  rancid  in  three 
days  if  exposed  to  the  action  of  the  light,  while  unsteril- 
ized,  normal  butter  exposed  under  similar  conditions,  did 
not  change  in  five  months,  if  the  air  was  excluded  from 
it.  Duclaux2  has  proven  that  the  rancid  change  is  largely 
a  chemical  action  that  takes  place  in  butter-fat  where  it 
is  exposed  to  light  and  oxygen;  that  it  is  not  necessarily 
inaugurated  by  the  vital  functions  of  any  special  kind  of 
bacteria.  While  the  change  goes  on  in  most  cases  in  a 
purely  chemical  way,  there  are,  however,  certain  organ- 
isms that  are  able  to  hasten  this  process  if  they  are 
present  in  the  butter3 .  For  this  reason  a  soft  butter 
containing  considerable  quantities  of  buttermilk,  and 
therefore  rich  in  nitrogenous  material,  undergoes  a  rapid 
change  and  quickly  becomes  rancid. 

145.  Defects  due  to  manufacturing'  methods. 
There  are  other  defects  in  butter  that  are  also  attributa- 
ble to  other  than  bacterial  causes.  These  are  for  the  most 
part  due  to  errors  in  manufacture.  Thus,  mottled  or 
wavy  butter  is  generally  caused  by  the  uneven  distribu- 
tion of  the  salt.  White  specks  in  butter  are  often  pro- 
duced where  the  cream  is  allowed  to  ripen  for  too  long  a 
time,  or  where  curdled  milk  is  used  as  a  starter.  In  such 
cases,  the  curd  particles  remain  undissolved,  and  are  held 
in  the  butter,  where  they  appear  as  white  specks. 


1  Ritsert,  Inaug.  Diss.  Berne,  1890. 

*  Duclaux,  Le  Lait,  p.  34. 

3  Von  Klecki,  Cent.  f.  Bakt.,  15:  354. 


DIVERSITY 


Bacteria  in  Butter- Making.  145 

B.     BACTERIAL  DEFECTS  IN  BUTTER. 

146.  Lack  of  flavor.     Often  this  may  be  due  to  im- 
proper handling  of  the  cream  in  not  allowing  it  to  ripen 
far  enough,  but  some  times  it  is  impossible  to  produce  a  high 
flavor.     The  lack  of  flavor  in  this  case,  is  due  to  the  ab- 
sence of  the  proper  flavor- producing  organisms.     This 
condition  can  usually  be  overcome  by  the  addition  of  a 
proper  starter.     The  relation  between  flavor  and  desir- 
able bacteria  is  very  intimate,  and  troubles  of  this  kind 
usually  arise,  because  the  proper  forms  commonly  found 
in  the  cream  have  been  supplanted  by  other  species  that 
do  not  possess  the  ability  of  forming  these  aromatic  sub- 
stances so  necessary  in  sour  cream  butter. 

147.  Putrid  butter.     This  specific  butter  trouble  has 
been  observed  in  Denmark,  where  it  has  been  thoroughly 
studied  by  Jensen1.     Butter  affected  by  it  rapidly  ac- 
quires a  peculiar  putrid  odor  that  ruins  it  for  table  use. 
Sometimes,  this  flavor  may  be  developed  in  the  cream 
previous  to  churning.     Jensen  found  the  trouble  to  be 
due  to  several  different  putrefactive  bacteria.     One  form 
which  he  called  Bacillus  fcetidus  lactis,  a  close  ally  of  the 
common  feces  bacillus,  produced  this  rotten  odor  and  taste 
in  milk  in  a  very  short  time.     Fortunately,  this  organism 
was  easily  killed  by  a  comparatively  low  heat,  so  that 
pasteurization  of  the  cream  and  use  of  a  culture  starter 
quickly  eliminated  the  trouble,  where  it  was  tried. 

1 48 .  Turnip-flavored  butter.    Butter  sometimes  ac- 
quires a  peculiar  flavor  recalling  the  odor  of  turnips, 
rutabagas,  and  other  root  crops.     Often  this  trouble  is 
due  to  feeding,  there  being  in  several  of  these  crops, 
aromatic  substances  that  pass  directly  into  the  milk,  but 
in  some  instances  the  trouble  arises  from  bacteria  that 

1  Jensen,  Cent.  f.  Bakt.,  11:  409,  1891. 
10 — B. 


146  Dairy  Bacteriology. 

are  able  to  produce  decomposition  products1,  the  odor 
and  taste  of  which  strongly  recalls  these  vegetables. 

149.  "Cowy"  odor  in  butter.     Frequently  there  is 
to  be  noted  in  milk  a  peculiar  odor  that  resembles  that 
of  the  cow  stable  and  sometimes  the  odor  of  the  animal 
itself.     Usually  this  defect  in  milk  has  been  ascribed  to 
the  absorption  of  impure  gases  by  the  milk  as  it  cools, 
although  the  gases  and  odors  naturally  present  in  fresh 
milk  have  this  peculiar  property  that  is  demonstrable  by 
certain  methods  of  aeration.     Occasionally,  it  is  trans- 
mitted to  butter,  and  recently,  Pammel2  has  isolated  from 
butter,  a  bacillus  that  produced  in  milk  the  same  peculiar 
odor  so  commonly  present  in  stables. 

150.  Lardy  and  tallowy  butter.     The  presence  of 
this  unpleasant  taste  in  butter  may  be  due  to  a  variety  of 
causes.     In  some  instances,  improper  food  seems  to  be 
the  source  of  the  trouble;    then  again,  butter  exposed  to 
direct  sunlight  bleaches  in  color  and  develops  a  lardy 
flavor.3     In  addition  to  these,  cases  have  been  found  in 
which  the  defect  has  been  traced  to  the  action  of  bacteria. 
S torch4  has  described  a  lactic  acid  form  in  a  sample  of 
tallowy  butter  that  was  able  to  produce  this  disagreeable 
odor. 

151.  Oily  butter.     Jensen  has  isolated  one   of  the 
causes  of  the  dreaded  oily  butter  that  is  reported  quite 
frequently  in  Denmark.     The  specific  organism  that  he 
found  belongs  to  the  sour-milk  bacteria.     In  twenty-four 
hours  it  curdles  milk,  the  curd  being  solid  like  that  of 
ordinary  sour  milk.     There  is  produced,  however,  in  ad- 
dition to  this,  an  unpleasant  odor  and  taste  resembling 

1  Jensen,  Molk.  Ztg.,  6:  Nos.  5  and  6,  1892. 

8  Pammel,  Bull.  21,  Iowa  Expt.  Stat.,  p.  803. 

3  Fischer,  Hyg.  Rund,  5:  573. 

4Storch,  18th  Kept.  Danish  Agric.  Expt.  Stat.,  1890. 


Bacteria  in  Sutler-Making.  147 

that  of   machine  oil,  a  peculiarity  that  is  transmitted 
directly  to  butter  made  from  affected  cream. 

152.  Bitter  butter.     Now  and  then  butter  develops  a 
bitter  taste  that  may  be  due  to  a  variety  of  different  bac- 
terial forms.  In  most  cases,  the  bitter  flavor  in  the  butter 
is   derived   primarily   from  the  bacteria  present  in  the 
cream   or  milk.     Several  of   the  fermentations   of  this 
character  in  milk  are  also  to  be  found  in  butter.     In  ad- 
dition to  these  defects  produced  by  a  biological  cause, 
bitter  flavors  in  butter  are  sometimes  produced  by  the 
milk  being  impregnated  with  volatile,  bitter  substances 
derived  from  weeds. 

153.  Moldy  butter.     This  serious  defect  in  butter  is 
caused  by  the  development  of  the  common  mold  fungus 
(Penicillmm  glaucum)  on  the  inside  of  butter  packages. 
Where  tubs  are  made  from  green,  sappy  wood,  the  mold- 
spores  that    are   practically  everywhere,  find  favorable 
conditions  for  growth,  especially,  where  moisture  exists. 
The  result  is  that  as  the  wood  molds,  the  outer  layers 
of  the  butter  are  stained  and  at  the  same  time  acquire 
a  moldy  taste.     This  defect  can  be  easily  obviated  by 
steaming  the  tubs  thoroughly  before  packing.      Butter 
tubs  should  always  be  kept  in  a  cool,  dry  place  under 
conditions  unfavorable  for  the  growth  of  the  mold-spores. 


CHAPTER  X. 
BACTERIA    IN  THE  CHEESE  INDUSTRY. 

154.  Relation  of  bacteria  to  cheese.    The  processes 
of  cheese-making  are  much  more  affected  by  the  operation 
of  bacterial  causes  than  almost  any  other  dairy  industry. 
Cheese  contains  so  large  a  proportion  of   nitrogenous 
matter  that,  so  far  as  food  is  concerned,  bacteria  find  bet- 
ter conditions    for  development  than   in   butter.      The 
ripening  or  curing  of  cheese  is  a  fermentative  process  in 
which  bacteria  are  intimately  concerned.     Our  knowledge 
of  the  actual  changes  induced  by  these  organisms  is,  as 
yet,   quite  meager,  but  enough  has  already  been  deter- 
mined to  indicate  that  the  whole  industry  is  based  largely 
on  phenomena  of  ferment  action,  and  that  the  application 
of  bacteriological  principles  and  ideas  to  this  phase  of 
dairying  is  sure  to  yield  more  than  ordinary  results,  in 
explaining  in  a  rational  way,  the  reasons  underlying  many 
of  the  operations  of  this  industry. 

155.  Principles  of  cheese-making*.     The  general 
principle  of  cheese-making  is  to  precipitate  the  casein, 
and  then  press  this  into  a  more  or  less  solid  mass,"  allow- 
ing it  to  stand  for  varying  periods  of  time,  and  under 
various  conditions,  so  that  a  wide  range  is  presented,  in 
which  various  fermentative  actions  may  operate  to  pro- 
duce the  widely  different  kinds  of  cheese.      From  the 
same  kind  of  milk,  a  great  variety  of  different  cheeses 
can  be  prepared,  depending  upon  the  treatment  of  the 
milk  during  the  manufacture,  and  the  way  in  which  the 

cheese  is  handled  after  it  is  made. 

[148] 


Bacteria  in  the  Cheese  Industry.  149 

In  precipitating  the  casein,  the  fat  is  caught  in  the 
curd  particles,  and  is  thus  incorporated  in  the  cheese. 
This  curdling  of  the  casein  is  brought  about  in  two  ways. 

1.  Addition  of  rennet,  a  chemical  ferment  extracted 
from  calves'  stomachs1,  which  has  the  specific  property 
of  rendering  the  casein  of  milk  insoluble,  e.  g.,  cheddar 
and  Swiss  varieties  of  cheese. 

2.  Development  of  acid  until  the  casein  is  precipitated, 
e.  </.,  sour  milk,  cottage  cheese. 

By  far  the  larger  amount  of  cheese  is  made  in  the  first 
way.  The  various  kinds  manufactured  may  be  divided 
into  two  general  classes,  soft  and  hard  cheese,  depending 
upon  the  character  of  the  finished  product,  which  is  pro- 
duced by  the  kind  of  treatment  that  is  given  the  milk  in 
the  manufacture  of  the  cheese  and  its  subsequent  curing. 
By  far  the  larger  amount  of  cheese  made  in  this  country 
belongs  to  hard,  firm  type  of  cheese. 

156.  Summary  of  manufacturing' details.  In  ma- 
king cheese,  it  is  necessary  to  concentrate  the  casein  and 
adherent  fat  into  a  compact  mass,  containing  only  a  lim- 
ited amount  of  water.  The  addition  of  the  rennet  pre- 
cipitates the  casein  and  causes  it  to  shrink  in  volume.  In 
order  to  facilitate  this  process,  the  coagulated  casein  is 
cut  into  small  pieces,  thereby  giving  a  better  opportunity 
for  the  continued  shrinking  action,  and  so  expel  the  milk- 
serum  or  whey.  This  change  is  greatly  affected  by  the 
presence  of  acid  and  heat.  In  order  to  make  the  process 
uniform  from  day  to  day,  the  milk  is  held  or  ''ripened" 
until  a  certain  degree  of  acidity  is  reached  from  the  de- 
velopment of  the  bacteria  that  are  invariably  present  in 
the  milk.  The  curds  are  then  heated  to  a  varying  tem- 
perature, the  action  of  the  heat  being  to  further  expel 

1  In  Italy  this  ferment  is  sometimes  secured  from  artichokes  and 
is  used  in  cheese  manufacture. 


150  Dairy  Bacteriology. 

the  moisture  which  is  separated  from  the  curd  in  the 
whey.  If  a  rather  dry,  slow- curing  cheese  is  wanted, 
the  curds  are  cooked  in  the  whey  at  a  higher  tempera- 
ture, and  for  a  longer  time,  than  if  a  more  rapidly  cur- 
ing product  is  desired. 

The  bacteria  originally  present  in  the  milk  are  able  to 
grow  with  very  great  rapidity  under  these  conditions, 
and  the  result  is,  that  if  the  normal  milk  bacteria  (lactic 
acid  forms)  are  present,  there  will  be  a  steady  develop- 
ment of  the  acid  produced  by  the  decomposition  of  the 
milk-sugar.  This  development  of  the  acid  in  the  curd 
has  a  marked  effect  upon  the  character  of  the  green 
cheese. 

157.  Influence  of  bacteria  in  making"  cheese.  In- 
asmuch as  the  proper  development  of  acid  in  the  milk 
before  and  after  the  casein  is  precipitated  is  determined 
largely  by  the  kind  of  bacteria  that  may  happen  to  be 
present,  it  necessarily  follows  that  the  success  of  the  pro- 
cess is  governed  somewhat  by  the  original  condition  of 
the  milk  as  to  its  bacterial  life.  In  pure,  clean  milk,  the 
native  species  of  bacteria  seem  to  be  present  in  sufficient 
numbers,  so  that  the  proper  development  of  the  acid  in 
the  vat  is  rendered  possible,  but  it  not  infrequently  hap- 
pens that  the  milk  may  be  "off'7  in  flavor,  i.  e.,  the  nor- 
mal milk  organisms  may  be  supplanted  by  other  species 
of  a  less  desirable  character  that  have  gained  access  to 
the  milk  through  improper  handling.  There  is  consider- 
able competition  among  different  species  of  bacteria  when 
growing  in  same  fluid,  and  sometimes  they  antagonize 
each  other  so  strongly  by  reason  of  the  by-products  which 
are  formed  that  where  one  kind  is  in  abundance,  other 
species  are  unable  to  grow.  So,  if  certain  kinds  are  in- 
troduced, say  from  dirt  or  filth  gaining  access  to  the  milk, 
these  species  may  repress  the  growth  of  the  normal  milk 


Bacteria  in  the  Cheese  Industry.  151 

forms  to  such  an  extent  that  the  necessary  acid  is  not 
produced. 

If  bacteria  are  present  in  the  milk  that  are  able  to 
peptonize  or  digest  the  same,  a  not  inconsiderable  amount 
of  the  cheese-producing  solids  may  be  dissolved  and  be 
lost  in  the  whey.  Organisms  of  this  class  are  usually 
derived  from  dust  and  animal  filth.  By  the  use  of  the  fer- 
mentation or  curd  test  (182),  it  is  possible  to  detect  a 
condition  of  this  sort  in  the  milk. 

158.  Starters  in  cheese-making1.  Where  the  milk 
reaches  the  factorv  in  an  extra  sweet  condition,  it  is 
sometimes  advisable  to  add  a  starter  to  hasten  the  ripen- 
ing of  the  milk.  Usually  sour  milk  is  employed,  but 
with  any  starter,  great  care  should  be  taken  to  add  noth- 
ing that  will  introduce  into  the  milk  any  undesirable  taint. 

Where  "gassy  "  milks  prevail,  the  proper  development 
of  the  acid  is  also  interfered  with,  and  in  such  cases,  the 
addition  of  a  lactic  ferment  is  of  great  aid  in  repressing 
these  abnormal  fermentations. 

While  the  ordinary  starter  used  is  one  of  domestic 
origin,  as  skim  or  sour  milk,  within  recent  years  the 
attempt  has  been  made  to  introduce  pure  cultures1  of  va- 
rious lactic  acid-producing  forms.  These  cannot  be  used 
in  pasteurized  milk  as  is  the  case  in  butter-making,  but 
even  where  added  to  the  raw  milk,  they  exert  a  material 
effect.  By  their  use  the  necessary  impetus  is  given  to 
the  formation  of  acid,  and  the  development  of  this  re- 
tards the  growth  of  the  gas-forming  organisms.  The 
period  of  manufacture  is  also  materially  shortened.  Their 
use  should,  however,  be  carefully  governed,  as  sour  cheese 
is  apt  to  develop  if  employed  too  freely. 

Campbell2  has  also  used  the  lactic  starter  to  overcome 

1  Russell,  13th  Kept.  Wis.  Expt.  Stat.,  p.  108,  1896. 

2  Campbell,  North  Brit.  Agric.,  May  12,  1897. 


152  Dairy  Bacteriology. 

a  serious  cheese  defect  that  occurs  frequently  in  Scotland, 
viz.,  the  discoloration  of  cheese. 

The  use  of  stringy  or  slimy  whey  has  been  advocated  in 
Holland  for  some  years  as  a  means  of  overcoming  the 
tendency  toward  gas  formation  in  Edam  cheese  which  is 
made  from  practically  sweet  milk .  This  fermentation ,  the 
essential  feature  of  which  is  produced  by  a  culture  of 
Streptococcus  Hollandicus1 ,  develops  acid  in  a  marked 
degree,  thereby  inhibiting  the  production  of  gas. 

159.  Bacteria  in  rennet.     The  addition  of  rennet  to 
milk  has  no  appreciable  effect  on  the  development  of  the 
bacteria  in  the  same.     The  idea  has  been  advanced  that 
the  bacteria  in  the  rennet  extract  exert  a  considerable 
influence  on  the  cheese;  this  view  is,  however,  hardly 
tenable,  in  spite  of  the  fact  that  the  rennet  usually  con- 
tains large  numbers  of  organisms,  for  the  relative  amount 
of  rennet  that  is  added  is  so  small  that  this  does  not  per- 
ceptibly increase  the  germ  content  of  the  milk. 

It  is  possible  that  undesirable  bacteria  may  be  intro- 
duced into  the  milk  by  means  of  the  rennet  where  the 
same  is  contaminated  with  noxious  forms ;  but  these  cases 
are  relatively  rare .  This  might  apply  particularly  to  those 
cases  in  which  the  natural  rennets  are  used.  These  are 
preserved  in  salt,  alcohol,  or  boric  acid,  but  they  are 
never  free  from  bacteria.  Adametz2  found  ten  different 
species  and  from  640,000-900,000  bacteria  per  cc.  in 
rennets. 

160.  Green  cheese.     If  the  proper  amount  of  acid 
has  been  allowed  to  develop,  a  cheese  fresh  from  the 
press  will  present  a  close  uniform  texture,  that  in  the 
"green"  condition  is  tough  and  elastic.     This  rapidly 
changes  as  the  cheese  ripens,  and  with  this  change  there 
is  a  marked  increase  in  the  amount  of  bacterial  life. 

1  Weigmann,  Milch  Ztg.,  No.  50.  1889. 

2  Adametz,  Landw.  Jahr.,  18:  256. 

v 


Bacteria  in  the  Cheese  Industry. 


153 


Where  but  little  acid  is  developed  as  in  a  sweet  curd 
cheese  the  texture  is  open  and  porous.  The  numerous 
cavities  that  are  to  be  noted  arise  from  the  fact  that  the 
curd  particles  have  not  been  closely  matted  together. 
This  matting  or  cementing  of  the  curd  masses  is  due  to 
their  partial  solution  under  the  liquefying  influence  of 
the  acid  produced  by  the  bacteria.  Where  these  "me- 
chanical holes"  are  present,  gas  is  almost  invari- 
ably developed,  as  the  gas-producing  bacteria  that  are 
quite  susceptible  to  acid  are  not  inhibited  under  these 
conditions.  This  gas  finds  its  way  into  the  ready-made 
mechanical  holes,  greatly  distending  them  as  in  fig.  33. 


FIG.  33.  Showing  "  mechanical  holes  "  in  a  sweet  curd  cheese  and  the  disten- 
tion  of  the  same  by  gas. 

L  shows  appearance  of  a  sweet  curd  cheese  immediately  after  it  is  taken  from 
the  press.  The  "  mechanical  holes  "  are  produced  by  the  failure  of  the  curd  par- 
ticles to  cement,  owing  to  the  absence  of  acid. 

P  shows  appearance  of  a  duplicate  cheese  four  days  after  removal  from  press. 
The  gas  produced  by  bacterial  germs  fills  up  and  distends  these  "  mechanical 
holes,"  thus  causing  a  marked  distortion  in  shape  of  cheese  (huffing). 

161.  Physical  changes  in  ripening.  When  a  green 
cheese  is  taken  from  the  press,  the  curd  is  tough,  firm, 
but  elastic.  It  has  no  value  as  a  food  product  for  imme- 


154  Dairy  Bacteriology. 

diate  use,  because  it  lacks  a  desirable  flavor  and  is  not 
readily  digestible.  It  is  nothing  but  precipitated  casein 
and  fat.  In  the  course  of  a  short  time,  a  deep-seated 
change  occurs.  Physically  this  change  is  demonstrated 
in  the  modification  that  the  curd  undergoes.  Gradually 
it  breaks  down  and  becomes  plastic,  the  elastic,  tough 
curd  being  changed  into  a  softened  mass.  This  change 
in  texture  of  the  cheese  is  also  accompanied  by  a  marked 
change  in  flavor.  The  green  cheese  has  no  distinctively 
cheese  flavor,  but  in  course  of  time,  with  the  gradual 
change  of  texture,  the  peculiar  flavor  incident  to  ripe 
cheese  is  developed. 

The  characteristic  texture  and  flavor  are  susceptible  of 
considerable  modification  that  is  induced  not  only  by 
variation  in  methods  of  manufacture,  but  by  the  condi- 
tions under  which  the  cheese  are  cured.  The  amount  of 
moisture  incorporated  with  the  curd  materially,  affects 
the  physical  appearance  of  the  cheese,  and  the  rate  of 
change  in  the  same.  The  temperature  at  which  the  same 
is  ripened,  likewise  the  moisture  content  of  the  surround- 
ing air,  also  exert  a  marked  influence  on  the  ripening 
product.  To  some  extent  the  action  of  these  forces  is 
purely  physical,  as  in  the  gradual  loss  by  drying,  but 
they  also  affect  the  course  of  the  biological  changes  to  a 
certain  extent. 

162.  Chemical  changes  in  ripening1.  The  marked 
physical  change  that  occurs  during  ripening  also  records 
a  profound  chemical  action  in  the  composition  of  cheese. 
Coincident  with  the  softening  of  the  curd,  comes  a  change 
in  the  condition  of  the  casein.  The  hitherto  insoluble 
casein  is  gradually  transformed  into  soluble  substances 
(caseone  of  Duclaux,  or  caseogluten  of  Weigmann) .  This 
chemical  phenomonon  is  a  breaking  down  process  that  is 
analogous  to  the  peptonization  of  proteids,  although  in 


Bacteria  in  the  Cheese  Industry.  155- 

addition  to  peptones,  numerous  other  intermediate  prod- 
ucts are  formed  ranging  from  albumens  to  ammonia. 
Amido  acids  (leucin  and  tyrosin)  are  present  in  an  old 
cheese.  The  chemical  reaction  of  cheese  is  generally 
slightly  acid  to  phenolphthalein,  although  in  an  old  cheese 
where  putrefactive  changes  are  going  on,  the  reaction 
may  be  alkaline. 

The  changes  that  occur  in  a  ripening  cheese  are  for  the 
most  part  confined  to  the  proteids.  The  fat  remains 
practically  unchanged.  In  the  green  cheese  considerable 
milk-sugar  is  present,  but  as  a  result  of  the  fermentation 
that  occurs,  this  is  rapidly  converted  into  acid  products. 

163.  Bacterial  flora  of  cheese.  It  might  naturally 
be  expected  that  the  green  cheese,  fresh  from  the  press 
would  contain  practically  the  same  kind  of  bacteria  that 
are  in  the  milk,  but  a  study  of  cheese  shows  a  peculiar 
change  in  the  character  of  the  flora.  In  the  first  place, 
fresh  cottage  cheese  made  by  the  coagulation  of  the 
casein  through  the  action  of  acid,  has  a  more  diversified 
flora  than  cheese  made  with  rennet,  for  the  reason,  as 
given  by  Lafar, 1  that  the  fermentative  process  is  farther 
advanced. 

When  different  varieties  of  cheese  are  made  from  milk 
in  the  same  locality,  the  germ  content  of  even  the  ripened 
product  has  a  marked  similarity,  as  is  illustrated  by 
Adametz's  work2  on  Emmenthaler  or  Swiss  hard  cheese, 
and  Schweitzer  Hauskase,  a  soft  variety.  Of  the  nine 
species  of  bacilli  and  cocci  found  in  mature  Emmenthaler, 
eight  of  them  were  also  present  in  ripened  Hauskase. 

Different  investigators  have  studied  the  bacterial  flora 
of  various  kinds  of  cheese,  but  as  yet  no  systematic 
comparative  work  has  been  done  in  an  extended  manner. 

1  Lafar,  Technical  Mycology,  p.  216. 
8Adametz,  Landw.  Jahr.,  18:  228. 


156  Dairy  Bacteriology. 

Freudenreich,1  and  the  writer,2  independently,  have 
determined  the  character  and  numbers  of  bacteria  in 
Emmen thaler  and  American  cheddar  cheese.  Their  re- 
sults practically  agree  as  to  the  course  of  the  bacterial 
changes  in  these  two  kinds  of  cheese. 

First,  there  is  a  marked  decrease  in  numbers  lasting 
for  a  day  or  so.  This  is  then  followed  by  an  enormous 
numerical  increase  that  is  entirely  produced  by  the  marked 
development  of  the  lactic  acid  forms.  Synchronous  with 
this  increase,  the  peptonizing  and  gas-producing  bacteria 
gradually  disappear,  so  that  in  a  short  period  of  time,  the 
bacterial  flora  seems  to  be  made  up  very  largely  of  this 
lactic  type  of  organisms.  This  rapid  development  is 
followed  by  a  gradual  decline  that  continues  in  an  old 
cheese  for  two  years  or  more.  As  the  cheese  dries  out, 
the  conditions  become  unfavorable,  and  as  the  lactic  acid 
species  are  not  able  to  form  spores  they  soon  succumb  to 
their  unfavorable  environment.  The  following  data 
illustrate  this: 

Bacteria  per  gram  in  cheddar  cheese  at  different  stages  of 
ripening . 

Date  of  analysis.  Total  number.    Lactic  acid  bacteria. 

Curd 26,532,000  22,560,000 

ist  day 21,060,000  20,176,000 

5th  day 43,716,000  39,680,000 

loth  day 98,080,000  97,280,000 

i9th  day 48,140,000  48,000,000 

36thday.... 11,171,000  11,000,000 

This  condition  has  been  found  to  exist  not  only  in 
American  cheddar  cheese  but  in  Canada3  and  England4. 


1  Freudenreich,  Landw.  Jahr.  d.  Schweiz,  4:  17;  5:  16. 

2  Russell,  13th  Wis.  Expt.  Stat.,  p.  95,  1896. 

3  F.  C.  Harrison  (unpublished  data). 

*  Lloyd,  Bath,  and  West  of  Eng.  Soc.  Kept.,  2:  180,  1892. 


Bacteria  in  the  Cheese  Industry.  157 

1 64.  Relation  of  curing1  temperature  to  bacterial 
flora.     It  has  long  been  known  that  a  variation  in  tem- 
perature, particularly  if  the  temperatures  exceeded  70° 
P.,  had  a  detrimental  effect  on  the  quality  of  the  cheese 
ripened  under  these  conditions.     The  cause  of  this  dif- 
ference is  not  thoroughly  apprehended  at  the   present 
time,  although  the  general  belief  is  that  it  is  due  to  the 
growth  of  various  kinds  of  bacteria  that  are  able  to  de- 
velop under  these  various  temperature  conditions. 

A  detailed  study  made  by  the  writer1  of  the  character- 
istics of  the  bacterial  flora  of  cheese  ripened  at  different 
constant  temperatures  shows  that  the  bacterial  life  is 
considerably  influenced  by  temperature  conditions.  The 
rise  and  fall  of  bacteria  in  cheese  ripened  at  normal  tem- 
peratures has  just  been  stated.  In  cheese  ripened  at  re- 
frigerator temperatures  (50-56°  F. ) ,  the  bacterial  flora  re- 
tains for  a  considerable  period  the  same  general  aspect  as  in 
the  milk.  In  this  respect  it  differs  somewhat  from  that  of 
normal  cheese,  in  which  a  differentiation  soon  takes  place, 
the  lactic  acid  bacteria  gaining  the  ascendency.  In  cheese 
cured  at  a  high  temperature  (80-86°  F.),  the  number  of 
bacteria  per  gram  is  greatly  diminished,  and  the  organ- 
isms fail  to  persist  for  as  long  a  period  of  time  as  under 
normal  conditions.  Some  difference  in  character  of  or- 
ganisms is  found  in  cheese  kept  at  low  and  high  temper- 
atures, but  this  is  not  always  constant  nor  as  well  marked 
as  has  been  supposed. 

165.  Theories  of  cheese  curing".     The  curing  of 
cheese  is  far  from  being  a  simple  problem,  yet,  within 
the  last  few  years  definite  knowledge  of  the  processes  con- 
cerned has  been  greatly  increased. 

In  the  earlier  theories  of  cheese-ripening  it  was  thought 
to  be  purely  a  chemical  change,  but  with  the  growth  of 
1  Russell,  14th  Wis.  Expt   Stat.,  p.  203,  1897. 


158  Dairy  Bacteriology. 

bacteriological  science,  evidence  was  forthcoming  that  in- 
dicated that  it  was  due  to  the  activity  of  organisms. 
Schaffer1  showed  that  if  milk  was  boiled  and  made  into 
•cheese,  it  failed  to  ripen.  Adametz2  added  to  greeh 
cheese  various  disinfectants  as  creolin,  thymol,  etc. 
He  found  that  this  practically  stopped  the  curing  process. 
From  these  experiments,  the  conclusion  was  drawn  that 
bacteria  must  be  the  cause  of  the  ripening.  As  to  the 
nature  of  the  organisms  that  were  supposed  to  be  con- 
cerned in  this  process,  the  evidence  is  not  convincing. 

166.  Digestive  or  peptonizing1  theory.  From  the 
chemical  products  determined  in  a  matured  cheese,  there 
can  no  longer  be  any  question,  but  that  the  general 
character  of  the  cheese-ripening  process  is  a  peptoniza- 
tion  or  digestion  of  the  casein.  Duclaux3  in  1887  pro- 
pounded the  theory  that  this  change  was  due  to  that  type 
of  bacteria  that  are  able  to  liquefy  gelatin,  peptonize 
milk,  and  cause  a  hydrolytic  change  in  proteids.  To  this 
widely  spread  group  that  he  found  in  cheese,  he  gave  the 
name  tyrotlirix  (cheese  hairs).  According  to  him,  these 
organisms  do  not  function  directly  as  ripening  agents, 
but  they  secrete  an  enzyme  or  unorganized  ferment  to 
which  he  applies  the  name  casease.  This  ferment  acts 
upon  the  casein  of  milk,  converting  it  into  a  soluble 
product  known  as  caseone.  These  organisms  grow  in 
liquid  milk  with  great  rapidity  and  if  they  function  as 
casein  transformers,  one  would  naturally  expect  to  find 
them  at  least  frequently,  if  not  predominating  in  the 
ripening  cheese,  but  such  is  not  the  case.  In  typical 
cheddar  cheese,  however,  they  rapidly  disappear,  although 
in  the  moister,  softer  varieties,  they  persist  for  consider- 

1  Schaffer,  Milch  Ztg.,  p.  146,  1889. 
8  Adametz,  Landw.  Jahr.,  18:  261. 
3  Duclaux,  Le.  Lait,  p.  213. 


Bacteria  in  the  Cheese  Industry.  159 

able  time.  Even  where  added  in  large  numbers  to  the 
curd,  they  soon  perish. 

While  the  by-products  of  proteid  decomposition  induced 
by  this  class  of  bacteria  are  very  similar  to  those  found 
in  a  well-ripened  cheese,  still  the  above  facts  have  seemed 
an  insuperable  barrier  to  the  general  acceptance  of  this 
explanation . 

167.  Lactic  acid  theory.  It  has  already  been  shown 
that  the  lactic  acid  species  seem  to  find  in  the  green  cheese, 
the  optimum  conditions  of  development,  that  they  increase 
enormously  in  the  cheese  for  a  short  period,  and  then 
finally  decline  in  numbers.  This  marked  development 
coincident  with  the  breaking  down  of  the  casein,  has  led 
to  the  view  that  has  been  so  ably  expounded  by  Freud- 
enreich1  that  this  type  of  bacterial  action  is  concerned 
in  the  ripening  of  cheese.  This  group  of  bacteria  are 
unable  to  liquefy  gelatin,  or  digest  milk,  or,  in  fact,  to 
exert  under  ordinary  conditions  any  proteolytic  or 
peptonizing  properties.  This  has  been  the  stumbling- 
block  to  the  acceptance  of  this  hypothesis  as  an  ex- 
planation of  cheese-ripening.  Freudenreich  has  recently 
carried  on  experiments  that  he  believes  solves  the  prob- 
lem. By  growing  cultures  of  these  organisms  in  milk 
to  which  sterile,  freshly  precipitated  chalk  had  been 
added,  he  was  able  to  prolong  the  development  of  bac- 
teria for  a  considerable  period  of  time,  and  as  a  result 
of  this,  he  finds  that  an  appreciable  part  of  the 
casein  is  digested.  Weigmann2  inclines  to  the  view  that 
the  lactic  acid  bacteria  are  not  the  true  cause  of  the  pep- 
tonizing process,  but  that  their  development  guides  or 
directs  the  character  of  the  ripening  by  giving  favorable 
conditions  for  the  development  of  other  species. 

1  Freudenreich,  Landw.  Jahr.  d.  Schweiz,  p.  85,  1897. 

2  Weigmann,  Cent.  f.  Bakt.,  II.  Abt.,  4:  593,  1898. 


160  Dairy  Bacteriology. 

168.  Enzyme  theory.      In  1897,  Babcock  and   the 
writer1  were  able  to  demonstrate  that  milk  contains  cer- 
tain inherent  unorganized  ferments  or  enzymes  that  have 
the  power  of  digesting  the  casein  of  milk,  when  kept  under 
the  influence  of  chemicals  that  would  suppress  bacterial 
action,   but  which  would  not  materially  interfere  with 
enzyme  activity.     This  ferment  they  find  present  in  the 
milk  of   different  species  of   mammalia   (burro,  horse, 
sheep,  goat,  pig,  buffalo,  and  human),  and  in  cows'  milk 
in  such  quantities  that  they  can  isolate  it  from  centrifuge 
slime.     The   effect  of  this  chemical  ferment  which  they 
call  galactase,  is  shown  in  the  breaking  down  of  the  nitro- 
genous matter  in  boiled  milk,  forming  albumoses,  pep- 
tones, amido  acids,  and  ammonia,  products  that  agree 
in  every  way  with  those  present  in  a  normally  ripening 
cheese.     This  ferment  is  allied  to  trypsin,  the  pancreatic 
enzyme,  and  Jensen2  has  recently  shown  that  pancreatic 
extracts  when  added  to  cheese  accelerate  the   digestive 
changes. 

169.  Present  status  of  theory  of  cheese-ripening-. 
Comparative  experiments  on  milk  and  cheese  seem  to 
show  beyond  all  question  that  the   breaking  down  of 
casein  into  peptone  and  related  products  is  due  to  the  ac- 
tion of  this  inherent  ferment  of  the  milk  which  is  called 
galactase,  and  not  to  the  action  of  bacteria  as  heretofore 
believed.     Previous  investigators  have  failed  to  recog- 
nize the  presence  of  this  ferment  in  milk  because  they 
have  sterilized  the  same  by  heat,  and  under  these  condi- 
tions galactase  is  destroyed.     A  brief  exposure  of  milk  to 
176°  F.  is  sufficient  to  accomplish  this  result,  and  even 
exposures  to  considerably  lower  temperature  weakens  its 
activity  considerably,  especially  if  the  reaction  of  medium 

1  Babcock  and  Russell,  14th  Wis.  Expt.  Stat.,  p.  161,  1897. 
8  Jensen,  Cent.  f.  Bakt.,  II.  Abt.,  3:  752. 


Bacteria  in  the  Cheese  Industry.  161 

is  acid.  This  undoubtedly  explains  the  contradictory  re- 
sults obtained  in  the  ripening  of  cheese  made  from  pas- 
teurized milk,  such  cheese  occasionally  breaking  down 
but  in  an  abnormal  manner. 

The  previous  results  as  to  the  failure  of  cheese  to  ripen 
when  treated  with  disinfectants, — experiments  which  were 
supposed  to  be  the  foundation  of  the  bacteriological 
theory  of  cheese-ripening  —  are  now  explainable  on  a 
new  basis.  The  casein  is  not  peptonized  because  these 
strong  disinfectants  destroy  the  activity  of  the  enzyme 
as  well  as  the  bacteria.  Where  chemicals  such  as  ben- 
zol, chloroform,  ether,  and  toluol  are  used,  bacterial  growth 
is  inhibited,  but  enzyme  action  is  still  possible.  In  the 
presence  of  these,  green  cheese  undergoes  the  normal 
peptonization  of  the  casein. 

1 70.  Flavor  of  cheese.  While  the  action  of  this  fer- 
ment is  exerted  on  the  decomposition  of  the  casein,  it  is 
not  yet  known  what  its  relation  is  to  the  production  of 
proper  flavors  in  cheese.  Acting  as  it  does  on  the  nitrog- 
enous compounds  of  the  milk,  it  has  a  marked  effect  on 
the  texture  of  the  ripened  cheese. 

The  manner  in  which  the  peculiar  flavors  that  charac- 
terize the  various  kinds  of  cheese  are  produced  is  yet  an 
unsolved  problem.  The  view  that  is  most  generally  ac- 
cepted is  that  this  most  important  phase  of  cheese  cur- 
ing is  dependent  upon  bacterial  activity,  but  the  organisms 
that  are  concerned  in  this  process  have  not  as  yet  been 
satisfactorily  determined.  In  a  number  of  cases,  differ- 
ent species  of  bacteria  have  been  separated  from  milk  and 
cheese,  that  have  the  power  of  producing  aromatic  com- 
pounds that  resemble  in  some  cases,  the  peculiar  flavors 
and  odors  that  characterize  some  of  foreign  kinds  of 
cheese;  but  an  introduction  of  these  into  curd  has  not 

ii— B. 


162  Dairy  Bacteriology. 

resulted  in  the  production  of  the  peculiar  variety,  even 
though  the  methods  of  manufacture  and  curing  were 
closely  followed.  The  similarity  in  germ  content  in  dif- 
ferent varieties  of  cheese  made  in  the  same  locality  has 
perhaps  a  bearing  on  this  question  of  flavor  as  related  to 
bacteria.  Of  the  nine  different  species  of  bacteria  found 
in  Emmen thaler  cheese  by  Adametz,  eight  of  them  were 
also  present  in  ripened  Hauskase.  If  specific  flavors  are 
solely  the  result  of  specific  bacterial  action,  it  might 
naturally  be  expected  that  the  character  of  the  flora  would 
differ. 

The  present  state  of  our  knowledge  does  not  permit 
any  definite  statement  being  made  with  reference  to  the 
cause  of  flavor- production  in  the  hard  type  of  cheese, 
such  as  cheddar  and  Emmenthaler. 

171.  Ripening1  of  moldy  cheese.  In  a  number  of 
foreign  cheese,  the  peculiar  flavor  obtained  is  in  part 
due  to  the  action  of  various  fungi  which  grow  in  the 
cheese,  and  there  produce  certain  by-products  that  flavor 
the  cheese.  Among  the  most  important  of  these  are  the 
Roquefort  cheese  of  France,  Stilton  of  England,  Gor- 
gonzola  of  Italy. 

Roquefort  cheese  is  made  from  goat's  or  cow's  milk, 
and  in  order  to  introduce  the  desired  mold,  which  is 
nothing  more  than  Penicillium  glaucum,  the  ordinary 
bread  mold,  carefully  prepared  moldy  bread  crumbs  are 
added  to  the  curd.  The  cheese  are  ripened  in  limestone 
caverns  where  the  temperature  and  moisture  content  are 
quite  constant.  When  partially  cured,  they  are  placed 
in  a  machine  that  punches  them  full  of  small  holes. 
These  slender  canals  allow  the  mold  organism  to  pene- 
trate the  whole  mass  more  thoroughly;  the  moldy  straw 
matting  upon  which  the  ripening  cheese  are  placed  help- 
ing to  furnish  an  abundant  seeding  of  the  desired  germ. 


Bacteria  in  the  Cheese  Industry.  163 

When  new  factories  are  constructed  it  is  of  advantage 
to  introduce  this  necessary  germ  in  quantities,  and  the 
practice  is  sometimes  followed  of  rubbing  the  walls  and 
cellars  of  the  new  location  with  material  taken  from  the 
old  established  factory.  In  this  custom,  developed  in 
purely  an  empirical  manner,  is  to  be  seen  a  striking  illus- 
tration of  a  bacteriological  process  crudely  carried  out. 

In  the  Stilton  cheese,  one  of  the  highly  prized  moldy 
cheese  of  England,  the  desired  mold  fungus  is  intro- 
duced into  the  green  cheese  by  exchanging  plugs  taken 
with  a  cheese  trier  from  a  ripe  Stilton. 

In  all  probability,  the  breaking  down  of  the  casein  is 
attributable  in  part,  at  least,  to  the  action  of  the  inherent 
ferments  of  the  milk,  although  with  some  of  the  softer 
cheese,  this  may  be  aided  by  bacterial  action. 

172.  Ripening1  of  soft  cheese.  The  type  of  ripen- 
ing which  takes  place  in  these  softer  cheese  is  materially 
different  from  that  which  occurs  in  the  first  class.  In 
many  cases,  the  peptonizing  action  does  not  go  on  uni- 
formly throughout  the  cheese  but  is  hastened  on  the  ex- 
terior by  the  development  of  organisms  that  exert  a 
solvent  effect  on  the  casein.  For  this  reason,  soft  cheeses 
are  usually  made  up  in  small  sizes,  so  that  this  action 
may  be  facilitated.  The  bacteria  that  take  part  in  this 
process  are  those  that  are  able  to  form  unorganized  fer- 
ments (similar  in  their  action  to  trypsin,  galactase,  etc.) , 
and  these  soluble  ferments  gradually  diffuse  from  the 
outside  through  the  cheese. 

Most  of  these  peptonizing  bacteria  are  hindered  in  their 
growth  by  the  presence  of  lactic  acid,  so  that  in  many 
cases,  the  appearance  of  the  digesting  organisms  on  the 
surface  is  delayed  until  the  acidity  of  the  mass  is  re- 
duced to  the  proper  point  by  the  development  of  other 
organisms,  principally  molds,  which  prefer  an  acid  sub- 


164  Dairy  Bacteriology. 

stratum  for  their  growth.  In  Brie  cheese  a  blue  coating 
of  mold  develops  on  the  surface.  In  the  course  of  a  few 
weeks,  a  white  felting  appears  which  later  changes  to 
red.  This  slimy  coat  below  the  mold  layer  is  made  up 
of  diverse  species  of  bacteria  and  fungi  that  are  able  to 
grow  after  the  acid  is  consumed  by  the  blue  mold.  The 
red  coating  acts  upon  the  casein,  also  producing  an  alka- 
line reaction  that  is  unfavorable  for  the  growth  of  the 
blue  mold.  Two  sets  of  organisms  are  therefore,  essen- 
tial in  the  ripening  process,  one  preparing  the  soil  for  the 
ferment  that  later  produces  the  requisite  ripening 
changes.  The  process  as  carried  on  is  largely  empirical 
and  if  the  red  coat  does  not  develop  at  the  proper  time, 
the  maker  resorts  to  all  sorts  of  devices  to  bring  out  the 
desired  ferment.  The  appearance  of  the  right  form  is 
dependent,  however,  upon  the  proper  reaction  of  the 
cheese,  and  if  this  is  not  suitable,  the  wished-for  growth 
will  not  appear. 

In  those  cases  where  the  ripening  change  takes  place 
from  outside  toward  the  center,  there  is  always  an  outer 
layer  that  is  changed  to  such  an  extent  as  to  be  unfit  for 
food. 

B.     BACTERIA  IN  ABNORMAL  CHEESE  PROCESSES. 

173.  Susceptibility  of  cheese  to  abnormal  changes. 

Cheese  more  than  butter  is  subject  to  detrimental  fermen- 
tations incited  by  bacteria,  because  it  is  so  much  better 
adapted  for  germ  growth  on  account  of  its  better  and 
more  available  food- supply.  Many  abnormal  ferments 
that  find  their  way  into  the  milk  before  it  is  made  up  de- 
velop these  peculiarities,  especially  in  cheese.  Then  too, 
the  method  of  curing  is  so  often  without  control,  es- 
pecially with  many  of  those  methods  that  employ  fungi 
as  agents  in  the  curing  process.  Inability  to  check  the 


Bacteria  in  the  Cheese  Industry.  165 

ripening  at  the  proper  moment  often  results  in  the  de- 
composition processes  being  carried  too  far,  and  in  this 
way  unpleasant  flavors  are  developed.1 

174.  Indefinite  faults  in  Cheddar  cheese.  Besides 
a  number  of  more  or  less  distinct  fermentations  of  diverse 
character,  ill- defined  defects  in  flavor  and  odor  are  often 
to  be  noted.  Cheese  possessing  these  maybe  thoroughly 
ripened  and  therefore  digestible,  but  their  market  value 
is  much  diminished  by  reason  of  these  improper  flavors. 
In  many  cases,  troubles  of  this  sort  are  undoubtedly 
traceable  to  faulty  manufacture,  but  more  often,  the  de- 
fects arise  from  the  presence  of  some  fermenting  organ- 
ism that  is  the  cause  of  the  unpleasant  flavor.  Our 
knowledge  of  the  influence  that  the  different  species  have 
in  the  ripening  changes  of  cheese  is  still  too  meager  to 
enable  us  to  trace,  in  all  cases,  these  undesirable  condi- 
tions to  their  proper  source.  The  great  majority  of  the 
taints  observed  in  the  factory  are  due  to  the  abnormal 
development  of  some  of  these  forms  capable  of  evolving 
unpleasant  or  even  putrid  odors.  Most  of  them  are 
seeded  in  the  milk  before  it  comes  to  the  factory  and  are 
due  to  careless  manipulation  of  the  milk  while  it  is  still 
on  the  farm.  Others  gain  access  to  the  milk  in  the  fac- 
tory, owing  to  unclean  conditions  of  one  sort  or  another. 
Sometimes  the  cheese-maker  is  able  to  overcome  these 
taints  by  vigorous  treatment,  but  often  they  pass  on 
into  the  cheese  only  to  detract  from  the  market  value  of 
the  product. 

In  studying  the  effect  of  the  different  organisms  found 
in  milk  in  their  relation  to  cheese-making,  we  have  iso- 
lated a  number  of  different  species  that  are  concerned  in 

1  Adametz  has   brought  together  in  his  little   book   entitled,   The 
causes  of  the  abnormal  ripening  of  cheese,  a  large  amount  of  data  that 
ertains  especially  to  European  conditions. 


166  Dairy  Bacteriology. 

the  production  of  these  "off"  flavors  in  the  curd.  If 
these  organisms  are  seeded  in  pasteurized  milk  and  the 
infected  milk  made  into  cheese,  they  develop  odors  that 
are  sometimes  noted  in  factories  troubled  with  taints. 
These  indefinite  "  off 7 '  flavors  and  odors  are  often  in- 
sufficiently pronounced  to  be  given  a  name,  consequently, 
it  will  be  impossible  to  describe  them  under  any  well  de- 
fined head. 

175.  " Gassy"  fermentations  in  cheese.    One  of 

the  worst  and  at  the  same  time  most  common  troubles  in 
cheese-making  is  where  the  cheese  undergoes  a  fermenta- 
tion marked  by  the  evolution  of  gas.  The  presence  of 
gas  is  recognized  by  the  appearance  either  of  spherical 
or  lens-shaped  holes  of  various  sizes  in  the  green  cheese; 
often  they  appear  in  the  curd  even  before  it  is  put  to 
press.  Usually  in  this  condition,  the  curds  look  as  if 
they  had  been  finely  punctured  with  a  pin,  and  are  known 
as  "pin-hole"  curds.  Sometimes  the  gas  holes  are 
larger,  even  approaching  the  large  round  so-called  ' '  Swiss 
holes."  When  the  gas  is  abundant,  the  holes  are  more 
apt  to  be  restricted  in  size.  The  formation  of  gas  may 
continue  to  such  an  extent  that  the  curd  even  floats  on 
the  surface  of  the  whey  before  it  is  removed.  These 
"floating  curds"  are  permeated  through  and  through 
with  gas  bubbles,  giving  the  curd  a  "spongy' '  appearance. 
If  "gassy"  curds  are  put  to  press  in  this  condition, 
an  abnormal  change  usually  occurs  within  a  few  days. 
The  fermentation  goes  on  in  the  green  cheese  causing  it 
to  swell  or  ' '  huff, ' '  until  it  may  be  considerably  distorted. 
The  gas  diffuses  with  some  difficulty  in  the  new  cheese 
so  that  the  mass  rapidly  swells.  The  fermentation  may 
be  so  energetic  as  to  actually  cause  the  cheese  to  crack, 
owing  to  the  pressure  of  the  contained  gas.  In  the  se- 
vere types  of  this  gaseous  fermentation,  the  product  is 


Bacteria  in  the  Cheese  Industry.  167 

rendered  worthless,  but  even  where  the  development  is 
not  so  marked,  the  flavor  of  the  cheese  is  impaired  and 
the  market  value  diminished.  The  difficulty  may  occur  at 
almost  any  season  of  the  year,  but  the  trouble  is  most 
frequently  observed  in  the  late  summer  months.  So 
common  are  these  difficulties  that  they  may  be  regarded 
as  the  worst  cheese  troubles  with  which  our  cheese- makers 
have  to  contend.  -• 

This  same  defect  occurs  in  other  kinds  of  cheese,  but 
it  is  particularly  marked  in  those  that  are  made  from  rel- 
atively sweet  curd.  In  Emmenthaler,  or  Swiss  cheese, 
the  formation  of  large  gas  holes  is  called  "  blahen."  A 
closely  related  phenomenon  is  seen  in  ' '  nissler J '  forma- 
tion, where  very  numerous  small  holes  are  formed  (thou- 
sand eyes) . 

The  cause  of  the  difficulty  has  long  been  charged  to 
various  sources,  such  as  lack  of  aeration,  improper  feed- 
ing, retention  of  animal  gases,  etc.,  but  in  all  these  cases 
it  was  nothing  more  than  a  surmise.  Very  often  the 
milk  does  not  betray  any  visible  symptom  of  fermentation 
when  received,  and  the  trouble  is  not  to  be  recognized 
until  the  process  of  cheese- making  is  well  advanced. 

Recent  studies  from  a  biological  standpoint  have,  how- 
ever, thrown  much  light  on  this  troublesome  problem; 
and,  with  the  cause  of  these  fermentative  changes  more 
fully  recognized,  it  is  quite  probable  that  improved  meth- 
ods of  handling  milk  will  be  rapidly  introduced  that  will 
enable  the  maker  to  exclude  these  troubles. 

176.  Relation  to  living-  germs.  The  formation  of 
gas,  either  in  the  curd  or  after  it  has  been  put  to  press, 
is  due  entirely  to  the  breaking  down  of  certain  elements, 
such  as  the  sugar  of  milk,  under  the  influence  of  various 
living  germs.  This  trouble  is  then  a  type  fermentation, 
and  is  therefore,  much  more  widely  distributed  than  it 


168  Dairy  Bacteriology. 

would  be  if  it  was  caused  by  a  single  specific  organism. 
There  are  present  in  all  milks  a  few  bacterial  forms  that 
are  able  to  produce  a  varied  series  of  fermentations  in 
which  different  gases  may  be  given  off.  Among  these 
organisms  are  a  large  number  of  .the  bacteria,  although 
yeasts  and  allied  germs  are  often  present  in  milk  and  are 
likewise  able  in  some  cases  to  set  up  fermentative  changes 
of  this  sort.  In  these  cases  the  milk-sugar  is  decom- 
posed in  such  a  way  as  to  give  off  CO2  and  H,  and  in 
some  cases,  alcohol. 

According  to  Guillebeau  a  close  relation  exists  between 
those  germs  that  are  able  to  produce  an  infectious  inflam- 
mation (mastitis)  in  the  udder  of  the  cow  and  some  forms 
capable  of  gas  evolution.  Several  outbreaks  of  "gassy" 
milk  have  been  traced  directly  to  animals  suffering  from 
an  acute  inflammatory  condition  of  the  udder  in  which 
it  has  been  shown  that  the  organisms  producing  this  dis- 
ease were  the  direct  cause  of  the  gas  production  in  the 
milk. 

Gas-producing  bacteria  are  so  numerous  that  they  are 
almost  always  present  in  any  sample  of  milk,  especially 
if  it  is  a  little  old.  Bolley  and  the  writer  isolated  six 
different  species  from  a  single  sample  of  summer  milk. 
Even  in  winter  we  have  found  them  present  in  milk  in 
such  numbers  as  to  require  special  care  in  the  manufac- 
ture of  cheese.  Under  normal  conditions,  where  care  is 
taken  in  the  handling  of  the  milk,  they  are  not  usually 
found  in  large  numbers,  and  when  thus  numerically  re- 
stricted, are  doubtless  kept  under  subjection  by  the  com- 
petition of  numerous  other  bacteria  in  the  milk. 

These  fermentations  are  very  often  observed  in  foreign 
kinds  of  cheese,  especially  those  that  are  made  after  the 
sweet-curd  process.  Adametz1  has  collated  data  on  the 

1  Adametz,  Die  Ursachen  u.  Erreg,  d.  abnorm.  Reif.  Vorg.  b. 
Kaese,  p.  27. 


Bacteria  in  the  Cheese  Industry.  169 

European  forms  that  have  been  isolated,  and  has  found 
nineteen  bacterial  species  (five  cocci,  fourteen  bacilli) 
and  eight  varieties  of  yeast-like  fungi  that  are  gas-pro- 
ducing organisms. 

If  pasteurized  milk  is  seeded  with  a  pure  culture  of 
any  of  these  gas-forming  organisms  and  made  into 
cheese,  an  intense  fermentation  is  always  to  be  noted. 
This  often  appears  in  the  curd,  sometimes  even  before 
the  whey  is  drawn.  The  cheese  when  taken  from  the 


%^:P«Sw 


FIG.  34.  "  Gassy"  cheese.  The  milk  from  which  this  cheese  was  made  was 
first  pasteurized,  and  then  infected  with  a  gas-generating  bacillus.  Note  not 
only  the  "  spongy  "  texture,  but  the  distorted  form. 

press  develops  gas  rapidly,  causing  it  to  swell  and  a  cross 
section  of  it  at  this  stage  will  show  a  spongy  structure. 
If  the  cheese  is  left  intact,  the  fermentation  may  progress 
to  such  an  extent  that  the  pressure  of  the  contained  gas 
will  cause  the  rind  to  split  open.  This  fermentation  does 
not  last  for  more  than  a  few  days;  then  the  swelled 
cheese  sinks  as  the  gas  slowly  diffuses  throughout  its 
mass,  but  the  flavor  of  the  product  is  generally  impaired. 


170 


Dairy  Bacteriology. 


ill.    Gaseous    fermentations    in   Swiss   cheese. 

From  the  researches  of  v.  Freudenreich,  Baumann,  and 
Weigmann,  it  is  known  that  certain  gas-producing  forms 
invariably  play  a  role  in  the  normal  ripening  of  Einmen- 
thaler  cheese.  Sometimes  Swiss  cheese  is  devoid  of 
these  gas  holes,  or  "  eyes,"  in  which  case,  they  are  said 
to  be  "blind."  Such  cheese  may  ripen  well  but  their 
market  value  is  diminished  on  account  of  the  absence  of 
these  round  holes. 

The  size  of  the  gas  holes  as  they  appear  in  abnormal 
cheese  is  by  no  means  constant.  Neither  is  the  character 
of  the  hole  dependent  upon  any  specific  germ.  If  the 
gas-forming  bacteria  are  plenty  and  quite  evenly  dis- 
tributed the  holes  will  be  small  and  numerous  as  in 


FIG.  35.     A  block  Swiss  cheese,  showing  "  gassy  "  fermentation. 


nissler ' '  cheese.  Where  the  organisms  are  less  frequent 
and  develop  in  small  groups,  then  the  "  eyes"  are  much 
enlarged.  As  the  different  organisms  vary  in  their  in- 
tensity of  gas  formation,  this  too,  modifies  the  size  of  the 
gas  holes  to  a  marked  degree. 


Bacteria  in  the  Cheese  Industry.  Ill 

178.  Treatment  of  "  pin-holey  "  curds.  When  this, 
type  of  fermentation  appears  during  the  manufacture 
of  the  cheese,  the  maker  can  control  it  in  part  within 
certain  limits.  These  methods  of  treatment  are,  as  a  rule, 
purely  mechanical;  in  other  cases  they  are  to  place  the 
curds  or  the  green  cheese  under  such  conditions  as  to 
prevent  the  development  of  gas.  One  of  the  methods 
that  is  purely  mechanical  in  its  nature  is  the  piling  and 
turning  of  the  curds  and  then  the  subsequent  grinding  of 
the  same  in  a  curd  mill.  This  breaks  up  the  curd  layers 
and  allows  the  gas  to  escape.  After  the  gas  has  been 
forced  out,  the  curds  are  then  put  to  press  and  the  whole 
mats  down  into  a  compact  mass. 

Another  method  of  treatment  based  upon  bacteriologi- 
cal principles  is  the  addition  of  a  starter  to  induce  the 
formation  of  acid.  Where  acid  is  developed  as  a  result 
of  the  growth  of  the  lactic  acid  bacteria,  the  gas-produc- 
ing species  do  not  readily  thrive;  therefore,  the  develop- 
ment of  gas  may  be  controlled  in  part  in  this  way.  An- 
other reason  why  acid  aids  in  repressing  the  development 
of  gas  is  that  the  curd  particles  are  partially  softened  or 
digested  by  the  action  of  the  acid.  This  causes  them  to 
mat  together  more  closely  and  there  is  not  left  in  the 
cheese,  the  irregular  openings  where  the  curd  particles- 
fail  to  unite  and  in  which  the  developing  gas  finds  lodg- 
ment. 

Another  method  that  is  also  useful  with  these  curds  is 
to  employ  salt.  This  represses  gaseous  fermentations  and 
the  use  of  more  salt  than  usual  in  making  the  cheese  will 
very  often  restrain  the  production  of  gas.  Tendency  to 
form  gas  in  Edam  cheese  is  controlled  by  the  addition  of 
a  starter  prepared  from  slimy  whey  (lange  wei)  which  is 
caused  by  the  development  of  an  acid-forming  organism. 

The  temperature  at  which  the  cheese  is  cured  also  ma- 


172  Dairy  Bacteriology. 

terially  affects  the  production  of  gas.  At  high  curing 
temperatures,  the  gas-producing  bacteria  are  able  to  de- 
velop more  rapidly;  therefore,  more  trouble  is  experienced 
in  summer  than  at  other  seasons.  The  relation  of  tem- 
perature of  curing-room  to  the  ripening  of  cheese  is  a 
question  of  very  great  importance.  We  may,  roughly 
speaking,  divide  normal  temperatures  into  three  zones 
more  or  less  well  marked,  safe,  hazardous,  and  danger- 
ous. Where  the  temperature  exceeds  70°  F.,  the  ripen- 
ing of  the  cheese  is  apt  to  be  impaired.  At  these  high 
temperatures,  gas- generating  forms  are  apt  to  become 
prominent. 

179.  Detection  of  tainted  milks.  Taints  in  milk 
may  be  due  to  two  sets  of  causes.  They  may  be  pro- 
duced by  bacterial  agency  or  absorbed  directly  from  some 
pre-existing  source.  Those  belonging  to  the  first  class 
are  extremely  difficult  to  detect  at  the  weigh-can,  for  the 
taint  itself  may  not  be  pronounced,  while  the  organism 
causing  it  may  exist  in  the  milk  in  large  numbers. 
Taints  of  this  class  are  especially  dangerous,  for  they  are 
produced  by  vital  causes  which  may  further  develop,  and 
thereby  greatly  intensify  the  undesirable  condition. 
Taints  due  to  absorption  can,  if  prominent,  be  recognized 
by  sense  of  smell.  A  detection  of  these  is  facilitated  by 
warming  the  milk,  thus  permitting  the  contained  odors 
to  escape  more  readily. 

It  is  extremely  desirable  that  some  method  of  recog- 
nizing these  taints  of  microbic  origin  should  be  employed, 
but  thus  far,  no  rapid  way  has  been  devised  that  is  ap- 
plicable at  the  weigh-can.  It  is  possible,  however,  to 
treat  the  milk  by  making  what  is  known  as  a  fermenta- 
tion test,  and  thus  encourage  the  early  development  of  the 
contained  bacteria,  but  these  methods  are  only  of  use  in 
preventing  future  troubles. 


Bacteria  in  the  Cheese  Industry.  173 

180.  Fermentation   tests.     Fermentation    tests  are 
based  upon  the  principle  that  if  milk  is  held  at  a  moder- 
ately high  temperature,  approximating  the  blood  heat, 
the  bacteria  in  the  same  will  develop  rapidly.     Particu- 
larly is  this  true  with  taint-producing  organisms  as  these 
seem  to  develop  more  rapidly  at  the  higher  temperature. 

A  number  of  tests  have  been  devised  that  are  based 
upon  the  operation  of  the  same  general  bacteriological 
principle.  In  Walther's  lacto-fermentator  the  milk  is 
allowed  to  stand  in  bottles  or  glass  jars  until  it  sours1. 
To  hasten  this  process  it  is  incubated  in  a  warm  place. 
If  the  curd  formed  is  homogeneous  and  has  a  pure  acid 
smell,  the  milk  is  regarded  as  all  right.  If  it  floats  in  a 
muddy  serum,  is  full  of  gas  or  ragged  holes,  it  shows 
abnormal  conditions.  As  usually  carried  out,  no  special 
attempt  is  made  to  free  the  vessels  used  for  this  purpose 
from  various  kinds  of  bacteria.  Often,  too,  the  samples 
are  left  uncovered,  thus  permitting  infection  to  occur 
from  without.  The  utility  of  this  method  could  be  ma- 
terially increased  by  using  sterile  tubes  and  having  same 
protected  by  a.  plug  of  cotton  from  external  contamina- 
tion. 

A  few  drops  of  rennet  are  sometimes  added  to  the 
milk  that  is  incubated  in  order  to  enable  a  more  ready 
detection  of  the  gas  that  is  evolved.  The  rents  and  splits 
in  the  curdled  casein  are  rendered  more  evident  in  this 
way. 

181.  Casein  or  rennet  tests.     The  action  of  rennet 
on  milk  can  be  utilized  in  determining  its  value  for  cheese 
purposes.     If  a  given  quantity  of  rennet  solution  is  added 
to  a  definite  quantity  of  milk  at  a  certain  temperature, 
the  time  of  coagulation  will  give  some  indication  as  to 
the  character  of  the. milk.     Good  milks  ought  to  curdle 

1  This  same  principle  is  employed  in  Gerber's  test. 


174  Dairy  Bacteriology. 

quickly,  and  the  curd  should  be  uniform  and  firm.  If  it 
is  soft  or  tender,  and  does  not  curdle  within  the  usual 
time  (which  will  vary,  of  course,  with  the  proportion  of 
rennet  and  milk  used)  it  shows  an  abnormal  condition, 
due  generally  to  the  addition  of  water  or  chemical  sub- 
stances. Milk  from  cows  advanced  in  lactation  (strip- 
pers) not  infrequently  reacts  abnormally  with  the  rennet 
test.  This  test  rests  upon  the  chemical  rather  than  the 
"biological  characteristics  of  the  milk. 

182.  Wisconsin  curd  test.     The  curd  test  in  some 
respects  is  a  modification  of  the  fermentation  test,  yet 
the  method  is  sufficiently  different  in  detail  to  merit  sep- 
arate mention.     As  here  outlined  it  originated  at  the 
Wisconsin   Dairy  School  in   1895,   as    an   experimental 
method  of  studying  the  influence  of  gas- generating  bac- 
teria in  cheese-making1 . 

In  the  curd  test,  a  small  pat  of  cheese  is  made  in  a 
glass  jar  from  each  sample  of  milk.  The  advantage  of 
this  method  over  the  ordinary  fermentation  tests  lies  in 
the  fact  that  the  samples  of  milk  are  treated  in  a  way  so 
as  to  conform  more  closely  to  cheese  conditions,  inasmuch 
as  the  whey  is  removed,  thus  permitting  the  ready  recog- 
nition of  gas-production  in  the  curd.  Nearly  all  milks 
contain  some  bacteria  capable  of  decomposing  the  milk- 
sugar  and  producing  gas.  In  the  curd  test  this  slight 
evidence  does  not  appear,  but  if  the  milk  contains  an 
abnormally  high  content  of  gas- generating  bacteria,  suf- 
ficient to  injure  the  quality  of  the  milk  for  cheese-mak- 
ing, then  such  a  condition  becomes  evident  when  a  curd 
test  is  made. 

183.  Improvised  apparatus.    A  simple,  inexpensive 
apparatus  can  be  readily  made  by  any  operator  in  the 

1  I2th  Kept.  Wis.  Expt.  Stat.,  p.  148,  1895.  A  full  description  of 
this  test  is  given  in  Bull.  67,  Wis.  Expt.  Stat.,  June,  1898. 


Bacteria  in  the  Cheese  Industry. 


175 


following  way:  Take  a  number  of  pint  glass  fruit  jars 
and  sterilize  the  same  with  the  covers  in  boiling  water. 
Fill  the  bottles  two-thirds  full  with  milk  from  the  indi- 
vidual patron's  supply.  Place  the  bottles  in  a  wash  tub 
and  fill  the  same  with  warm  water.  The  temperature 
of  the  water  after  the  tub  is  filled  should  be  from  95-100° 
F.  When  the  milk  reaches  98°  F. ,  add  to  each  sample  by 
means  of  a  pipette,  ten  drops  of  rennet  extract  and  mix 
thoroughly.  Allow  the  jars  to  remain  undisturbed  until 
the  milk  is  thoroughly  curdled,  then  cut  the  curd  into 


FIG.  36.  Improved  bottles  for  making  curd  test.  A,  test  bottle  complete;  B, 
test  bottle  showing  construction  of  cover;  s,  sieve  to  hold  back  the  curd  when 
bottle  is  inverted;  c,  outer  cover  with  d  h  drain  holes  to  permit  of  removal  of 
whey. 

small  particles  with  a  case  knife,  stirring  the  same  to 
better  expel  the  whey.  Pour  off  the  whey  as  soon  as  the 
<3urd  settles,  repeating  this  process  at  frequent  intervals 
until  the  curd  mats.  The  excess  of  fermentable  sugar  is 
thus  removed.  Keep  the  temperature  of  water  at  blood 
heat  (98°  F.)  for  six  to  eight  hours,  and  then  examine 
the  cut  surface  of  the  curds. 


176  Dairy  Bacteriology. 

184.  Improved  apparatus.  A  more  convenient  form 
of  apparatus  has  been  devised  by  various  dairy  manufac- 
turers. This  consists  of  a  water-tight  box,  with  cover, 
which  is  provided  with  a  faucet  for  the  withdrawal  of  the 
water;  also  a  rack  to  hold  the  bottles.  The  test  bottles 
are  made  with  parallel  sides,  the  large  top  permitting  of 
the  easy  removal  of  the  curd.  The  tops  are  provided 
with  a  sieve  of  such  construction  that  the  bottles  will 
drain  thoroughly  if  inclined  in  an  inverted  position 
(fig.  36). 


FIG.  37.     Curd  from  a  good  milk.    The  large  irregular  holes  are  mechanical. 

185.  Interpretation  of  results  of  test.  The  curd 
from  a  good  milk  has  a  firm,  solid  texture,  and  should 
contain  at  most  only  a  few  small  pin  holes.  It  generally 
has  a  few  large,  irregular,  "mechanical"  holes  where  the 
curd  particles  have  failed  to  cement  as  is  seen  in  fig.  37. 
If  gas-producing  bacteria  are  very  prevalent  in  the  milk, 
the  conditions  under  which  the  test  is  made  cause  such 
a  rapid  growth  of  the  same  that  the  evidence  of  the  ab- 
normal fermentation  may  be  readily  seen  in  the  spongy 
texture  of  the  curd.  If  the  undesirable  organisms  are 
not  very  abundant  and  the  conditions  not  especially  suit- 


Bacteria  in  the  Cheese  Industry.  177 

able  to  their  growth,  only  small  "pin  holes"  may  appear; 
if,  on  the  contrary,  the  fermentative  process  takes  a  more 
violent  course,  such  as  would  be  the  case  in  a  "floating" 
milk,  then  the  test  curd  will  be  spongy  and  full  of  holes 
as  in  fig.  38.  Sometimes  the  curds  will  show  no  evidence 
of  gas,  but  their  abnormal  condition  can  be  recognized 
from  the  "mushy"  texture  and  the  presence  of  "off" 
flavors  that  are  materially  intensified  by  keeping  them 
in  closed  bottles.  The  odor  should  always  be  noted  with 
care,  as  this  is  of  more  importance  than  the  gas  which 
can  be  eliminated  to  a  large  extent  by  proper  manipula- 
tion during  the  manufacture  of  the  cheese. 


FIG.  38.     Curd  from  a  badly  tainted  milk.    Large  ragged  holes  mechanical 
small  "  pin  holes  "  due  to  gas. 

186.  Bitter  cheese.  Bitter  flavors  are  sometimes 
developed  in  cheese  especially  where  the  ripening  process 
has  not  been  fully  completed,  or  where  improper  temper- 
atures have  been  maintained  for  a  considerable  length  of 
time.  Several  organisms  associated  with  this  abnormal 
fermentation  have  been  noted. 

Guillebeau1  isolated  several  forms  from  Emmenthaler 

lGuillebeau,  Landw.  Jahr..  1890,  p.  27. 
12— B. 


178  Dairy  Bacteriology. 

cheese  which  he  connected  with  udder  inflammation  that 
were  able  to  produce  a  bitter  substance  in  cheese. 

Von  Freudenreich1  has  recently  described  a  new  form, 
Micrococcus  easel  amari  (micrococcus  of  bitter  cheese) 
that  was  found  in  a  sample  of  bitter  Swiss  cheese.  This 
germ  is  closely  related  to  Conn's  micrococcus  of  bitter 
milk.  It  develops  lactic  acid  rapidly,  coagulating  the 
milk  and  producing  an  intensely  bitter  taste  in  the  course 
of  one  to  three  days.  When  milk  infected  with  this  or- 
ganism is  made  into  cheese,  there  is  formed  in  a  few  days, 
a  decomposition  product  that  imparts  a  marked  bitter 
flavor  to  the  cheese. 


FIG.  39.     "  Floating  "  or  "  spongy  "  curd  from  a  very  bad  milk. 

It  is  peculiar  that  some  of  the  organisms  that  are  able 
to  produce  bitter  products  in  milk  do  not  retain  this 
property  when  the  milk  is  worked  up  into  cheese. 

187.  Putrid  or  rotten  cheese.  Sometimes  cheese 
undergoes  a  putrefactive  decomposition  in  which  the 
texture  is  profoundly  modified  and  various  foul- smelling 
gases  are  evolved.  These  often  begin  on  the  exterior  as 
small  circumscribed  spots  that  slowly  extend  into  the 

1  Freudenreich,  Fuehl.  Landw.  Ztg.,  43:  361. 


Bacteria  in  the  Cheese  Industry.  179 

cheese  changing  the  casein  into  a  soft  slimy  mass.  Then, 
again,  the  interior  of  the  cheese  undergoes  this  slimy  de- 
composition. The  soft  varieties  are  more  prone  toward 
this  fermentation  than  the  hard,  although  the  firm 
cheeses  are  by  no  means  exempt  from  the  trouble.  The 
verlauf  en  "  or  "  running ' '  of  limburger  cheese  is  a 
fermentation  allied  to  this.  It  is  where  the  inside  of  the 
cheese  breaks  down  into  a  soft  semi-fluid  mass.  In  se- 
vere, cases,  the  rind  may  even  be  ruptured,  in  which  case, 
the  whole  interior  of  the  cheese  flows  out  as  a  thick  slimy 
mass  having  sometimes  a  putrid  odor.  The  conditions 
favoring  this  putrid  decomposition  are  usually  associated 
with  an  excess  of  moisture,  and  an  abnormally  low  ripen- 
ing temperature. 

188.  Pigment  changes  in  cheese.  Occasionally 
with  hard  cheeses,  but  more  often  with  the  softer  foreign 
varieties,  abnormal  conditions  are  noted  that  express 
themselves  in  the  production  of  various  pigments  in 
the  different  sorts  of  cheese.  The  production  of  these 
variously  colored  pigments  are  due  mainly  to  the  action 
of  bacteria,  yeasts, -and  molds.  More  frequently  they  are 
merely  superficial,  and  affect  only  the  exterior  layers  of 
the  cheese. 

1.  Rusty  cheese.  This  difficulty  has  been  the  source 
of  considerable  trouble  to  makers  of  white  or  un- 
colored  cheddar  cheese  in  some  sections  of  America.  It 
is  characterized  by  the  appearance,  especially  in  July  and 
August,  of  rusty  red  or  orange  spots  that  are  noted 
throughout  the  interior  of  the  cheese,  particularly  along 
the  edge  of  the  curd  particles.  Council1  traced  an  out- 
break of  this  sort  in  a  Canadian  factory  to  defective 
drains,  and  succeeded  in  isolating  a  red  chromogenic 

1  Connell,  Kept,  of  Com.  of  Agric.  and  Dairying,  part  xvi;  p.  14, 

1898. 


180  Dairy  Bacteriology. 

bacillus  that  when  inoculated  into  milk,  and  cheese  made 
therefrom,  produced  the  rusty  discoloration.  A  similar 
defect  has  been  studied  by  Campbell1  in  Scotland.  He 
used  a  pure  lactic  ferment  with  good  results  in  over- 
coming the  tendency  to  discoloration. 

2.  Blue  cheese.     Blue  discoloration  in  cheese  sometimes 
arises  from  copper  and  iron  salts  derived  from  manufac- 
turing utensils.     Especially  is  this  liable  to  occur  with 
foreign  cheese  where  utensils  of   such  a  character  are 
used  in  the  manufacture  of  these  cheese.     De  Vries2  has 
described  a  blue  disease  in  Edam  cheese  that  he  ascribes 
to  the  action  of  bacteria.     It  appears  first  as  a  small  blue 
spot  on  the  inside,  increasing  rapidly  in  size  until  the 
whole  mass  is  often  affected.     By  the  use  of  slimy  whey, 
this  abnormal  change  is  controlled.3 

3.  Black  cheese.     Black  cheese  have  now  and  then  been 
noted,  especially  with  limburger  products.     This  appear- 
ance is  caused  by  the  copious  growth  of  different  forms 
of    low   fungi,   mainly  those    that   spread  out  in   tiny 
threads,  like  the  molds.     In  one  instance  a  yeast-like 
form  has  also  been  isolated.     So  far  as  known  troubles  of 
this  sort  are  not  caused  by  bacteria. 

189.  Molding*  of  cheese.  With  some  varieties"  of 
cheese,  more  especially  foreign  kinds,  the  presence  of  mold 
on  the  exterior  of  cheese  is  not  regarded  as  detrimental ;  in 
fact  the  limited  development  of  this  condition  is  much 
desired.  In  hard  rennet  cheese,  such  as  Emmenthaler 
and  Cheddar,  the  market  demands  a  product  free  from 
mold,  although  it  must  be  said  that  this  condition  is  im- 
posed by  the  desire  to  secure  a  good  looking  cheese  rather 
than  to  any  injury  in  flavor  that  the  mold  might  cause. 

1  Campbell,  North  Brit.  Agric.,  May  12,  1897. 

8  De  Vries,  Milch.  Ztg.,  1888,   Nos.  44,  45. 

3  Weigmann,  Cent.  f.  Bakt.,  II.  Abt.,  4:  607,  1898. 


Bacteria  in  the  Cheese  Industry.  181 

Development  of  mold  invariably  takes  place  in  a  moist 
atmosphere,  either  at  high  or  low  curing  temperatures. 
These  are  frequently  the  conditions  that  are  required 
in  curing  certain  types  of  hard  American  cheese,  so  that 
mold  is  apt  to  appear. 

The  use  of  a  double  bandage  or  rubbing  the  cheese 
when  ready  for  market  will  remove  this  blemish.  Recent 
experiments  with  certain  disinfectants  like  formalin  show 
that  SL  2  %  solution  rubbed  over  surface  will  retard  the 
development  of  mold. 

190.  Poisonous  cheese.  The  production  of  poison- 
ous ptomaines  in  cheese  is  perhaps  more  common  than 
with  almost  any  other  food  product.  Vaughan1  to  whom 
the  most  of  the  work  on  this  subject  is  to  be  credited, 
reports  over  300  cases  of  cheese-poisoning  in  two  years. 
It  seems  to  be  very  much  more  common  here  in  America 
than  it  is  in  Europe.  Vaughan  has  isolated  from  numer- 
ous samples  a  highly  poisonous  alkaloid  that  he  calls 
tyrotoxicon.  This  ptomaine  has  also  been  frequently 
demonstrated  in  milk,  cream,  and  ice  cream.  The  poi- 
sonous substances  formed  in  the  milk  are  probably  pro- 
duced through  the  agency  of  putrefactive  organisms  that 
gain  access  to  the  milk. 

The  poisons  secreted  in  the  milk  are  transferred  to  the 
cheese  with  their  virulence  often  unimpaired,  so  that  seri- 
ous complications  follow  where  the  infected  material  is 
used  for  human  food.  Vaughan  states  that  cheese  which 
reddens  litmus  paper  rapidly  should  be  regarded  as  sus- 
picious. In  normal  cheese,  this  reaction  occurs  very 
slowly.  Danger  from  this  source  is  to  be  noted  particu- 
larly with  the  so-called  sour  milk  cheese;  as  the  methods 
by  which  these  are  made  give  the  most  favorable  oppor- 

1  Vaughan,  Zeit.  f.  physiol.  Chemie,  10:  146. 


182  Dairy  Bacteriology. 

tunity  for  the  development  of  poisonous  decomposition 
products. 

191.  Prevention  of  cheese  defects.  In  attempting 
to  treat  the  various  defects  or  diseases  found  in  cheese, 
no  uniform  method  can  be  applied  in  all  cases.  Exclud- 
ing faults  due  to  manufacturing  methods,  most  of  the 
troubles  traceable  to  bacteria  originate  in  the  milk  before 
it  comes  to  the  factory. 

In  some  instances  the  maker  can  repress  or  subdue  the 
influence  of  these  undesirable  ferments  by  changing  his 
methods  somewhat  to  suit  the  varying  conditions.  But 
what  he  must  insist  upon  under  all  circumstances  is  that 
the  milk  shall  be  handled  by  the  patron  in  such  a  way  as 
to  exclude  as  far  as  possible  all  conditions  favoring  the 
access  of  any  organisms  into  it.  If  this  is  done  and  pains 
are  taken  to  point  out  to  the  individual  patron  the  way 
in  which  his  milk  becomes  infected,  then  the  maker  may 
expect  to  reap  the  reward  for  his  teachings  in  the  im- 
proved condition  of  the  milk. 


GLOSSARY. 


Adventive. — Species  introduced  from  foreign  sources. 

Aeration. — The  intimate  mixing  of  air  with  the  milk. 

Aerobes. — Bacterial  organisms  requiring  free  oxygen  for  growth. 

Algae. — Green  water  plants  of  simple  organization. 

Amido-acids. — Organic  acids  formed  in  proteid  decomposition. 

Anaerobes. — Bacteria  growing  without  free  oxygen. 

Anthrax  (splenic  fever). — A  contagious  animal  disease  character- 
ized by  blood  poisoning. 

Antiseptics. — Chemicals  capable  of  restraining  bacterial  growth. 

Bacillus  (plural  bacilli). — Straight,  rod-like  bacteria. 

Brownian  movement. — A  physical  movement  independent  of  vital 
force. 

Carbohydrates. — Organic  substances  like  sugars  and  starches. 

Casease.  — The  enzyme  capable  of  converting  casein  into   soluble 
compounds. 

Caseone. — Casein  that  has  been  rendered  soluble  by  enzymes. 

Ceil. — The  simplest  form  unit  of  vegetable  or  animal  life. 

Chlorophyll. — The  coloring  matter  in  leaves  of  green  plants. 

Chromogenic.— Color-producing. 

Cilia  (singular  cilium). — Tiny,  whip-like  protoplasmic   appendages 
of  cells,  serving  as  locomotor  organs. 

Coccus  (plural  cocci). — Bacteria  having  a  spherical  shape. 

Colony. — The  progeny  of  a  single  germ  growing  in  an  isolated  mass. 

Culture  medium. — A  food  substance  on,  or  in  which  bacteria  can 
grow. 

Culture  starters.  — Starters  for  use  in  butter  or  cheese-making  that 
are  developed  from  a  selected  bacterial  culture. 

Deodorants. — Chemicals  that  destroy  odors. 

Diastase. — The  starch-converting  enzyme. 

Disinfectant.  —  Any  chemical  substance  able  to  destroy  germ  life. 

Endospores. — Spore  formed  within  a  mother  cell. 

Enzymes. — Unorganized  chemical  ferments  not  endowed  with  life. 

Facultative. — Forms   that  possess  the  faculty  of   growing    under 
varied  conditions. 

Filmjolk. — Ropy  milk  of  Finland. 

[183] 


184  Glossary. 

Fission. — Division  of  a  cell  by  direct  partition. 

Formalin. — A  watery  solution  of  formic  aldehyde  gas  possessing 
powerful  disinfecting  properties. 

Galactase. — An  unorganized  ferment  inherent  in  milk  which  digests 
casein  in  ripening  cheese. 

Germicide. — Any  substance  capable  of  destroying  germ  life. 

Hydrophobia. — A  highly  contagious  disease,  often  occurring  in 
dogs,  wolves,  etc.,  but  communicable  to  man. 

Hydrocarbons. — Organic  material  like  the  fats  and  oils. 

Indigenous. — Bacteria  normal  to  any  given  habitat. 

Lange  wei  (slimy  whey). — A  viscous  fermentation  of  whey  used  as 
a  starter  in  Edam  cheese. 

Mammalia. — Animals  that  suckle  their  young. 

Mammitis. — Inflammation  of  the  udder. 

Matzoon. — A  fermented  milk  used  largely  in  Armenia. 

Mother  cell. — A  vegetable  cell  capable  of  reproduction. 

Obligate. — Bacteria  that  are  obliged  to  grow  under  certain  condi- 
tions. 

Optimum  growth  temperature. — The  best  temperature  for  the 
growth  of  any  form. 

Parasites. — Organisms  subsisting  on  living  matter. 

Pasteurization. — The  use  of  heat  from  140°  to  165°  F.  as  a  germ 
destroyer. 

Pathogenic. — Bacteria  able  to  produce  disease  in  living  tissue. 

Pepsin. — An  enzyme  capable  of  digesting  proteids  in  acid  solution. 

Peptones. — Soluble  products  of  nitrogenous  digestion. 

Preservaline. — A  proprietary  antiseptic  composed  mainly  of  boric 
acid. 

Proteids  — Complex  nitrogenous  substances  of  an  insoluble  nature. 

Protoplasm. — The  living  substance  of  organic  tissues. 

Ptomaine. — A  complex  nitrogenous  product  of  bacterial  growth. 

Pure  Culture. — A  bacterial  growth  of  a  single  species  in  a  sterile 
medium. 

Rennet. — The  curdling  enzyme  of  milk. 

Saccharomyces. — A  genus  of  plants  embracing  the  yeasts. 

Saprophytes. — Organisms  subsisting  on  dead  organic  matter. 

Sarcina. — Spherical  bacteria  forming  packets  of  cells. 

Spirillum  (plural,  spirilla). — Curved  or  bent  cylindrical  bacteria. 

Spores. — The  latent  or  resting  stage  of  certain  types  of  bacteria. 

Sterilization. — The  use  of  heat  in  the  neighborhood  of  212°  F.  as  a 
germ  destroyer. 


Glossary.  185 

Streptococcus. — Spherical  bacteria  growing  in  chains. 

Taettemjolk. — The  clotted  milk  of  Norway. 

Thermal  death-point. — The  temperature  at  which  the  cell  is  de- 
stroyed. 

Toxic. — Poisonous. 

Trypsin. — The  active  principle  of  the  pancreatic  secretion. 

Tyrotoxicon.  —A  poisonous  ptomaine  isolated  from  milk  or  cheese. 

Tyrothrix. — Digesting  bacteria  supposedly  concerned  with  cheese- 
ripening. 

Viscogen. — A  solution  of  lime  dissolved  in  sugar  used  in  restoring 
the  consistency  of  cream  that  has  been  heated  above  145°  F. 


INDEX. 


Abnormal  cheese  processes,  bacteria 
in,  164. 

Acid,  development  of,  in  butter-mak- 
ing, 129. 

Acid  test,   in  selecting  milk  for  pas- 
teurizing, 100. 
Method  of  using,  101. 

Action  of  bacteria  in  nature,  10. 

Aeration  of  milk,  45. 

Aerobic  bacteria,  7. 

Air:  bacteria  in,  12;  influence  in  milk 
contamination,  37. 

Alcoholic  fermentation  in  milk,  62,  63. 

Anaerobic  bacteria,  7. 

Animal,  influence  of,  on  milk,  32,  43. 

Anthrax,  82. 

Antiseptics,  10,  88. 

Apparatus:  cooling  milk  or  cream,  114; 
for  milk  testing,  174,  176;  pasteur- 
izing, 106,  108. 

Aroma:  of  butter,  129;  origin  of,  130. 

Bacillus;  definition  of,  2. 

acidi  lactici,  57;  colt  communis, 
59;  cyanogenus,  69;  foetidus  lactis, 
145;  lactis  erythrogenes,  68;  lactis 
saponacei,  67;  prodigiosus,  68; 
synxanthus,  70;  tuberculosis,  76, 
119. 

Bacteria:  action  in  nature,  10;  arrange- 
ment of  cells,  3;  in  buttermilk,  122; 
classification  of,  4;  in  cheese,  156; 
culture  of,  17;  in  cream,  128;  dis- 
covery of,  1;  external  conditions 
affecting,  8;  form  of,  2;  in  butter, 
142;  in  butter-making,  127;  in  cen- 
trifuge slime,  119;  in  fore  milk,  30; 
in  rennet,  152;  in  separator  slime, 
119;  isolation  of,  19;  manner  of 
growth,  3;  motility  of,  4;  nature  of, 
1;  parasitic,  13;  rapidity  of  multi- 
plication, 7;  reproduction  of,  3; 
saprophytic,  13;  size  of,  2;  speciali- 
zation of,  11;  structure  of,  2. 
Distribution  of:  air,  12,  37;  Boston 
milk,  50;  European  milk,  50;  in  ice, 


48;  in  milk,  49, 52;  milk  of  Madison, 
51;  milk  of  Middletown,  Ct.,  51; 
in  relation  to  cheese,  148;  in  soil, 
12;  in  water,  12,  48. 
Of  disease:  anthrax,  82;  cholera,  83; 
diphtheria,  84;  lockjaw,  82;  toxic, 
85;  tuberculosis,  76;  typhoid  fever, 
82,  126. 

Methods  of  study  of:  culture,  17; 
culture  media,  18;  isolation,  19; 
microscopy,  22. 

Bacterial  changes:  in  abnormal  cheese 
processes,  164;  in  butter,  142;  in 
cheese  manufacture.  152;  effect  in 
ripening  of  cream,  131. 

Bacterial  flora  in  cheese,  155. 

Bacteriological  study  of  pasteurized 
cream,  116. 

Barns,  a  source  of  infection,  44. 

Bitter  butter,  147;  cheese,  177;  fermen- 
tation in  milk,  64. 

Black  cheese, 180. 

Bloody  milk,  68. 

Blue  cheese,  180;  milk,  69. 

Boracic  acid  in  milk,  88. 

Bovine  tuberculosis,  77. 

Brie  cheese,  164. 

Butter:  bacteria  in,  142;  bitter,  147; 
"cowy,"146;  lardy,  146;  manufac- 
turing defects,  144;  moldy,  147;  oily, 
146;  putrid,  145;  rancid,  143;  ripened 
cream,  128;  sweet,  127;  tallowy,  147; 
turnip  flavor  in,  145. 
Making:  aroma,  129;  bacteria  in  ri- 
pening changes,  131;  development 
of  acid  in,  129;  flavor  in,  129;  from 
pasteurized  milk  and  cream,  134; 
lack  of  flavor,  145;  in  ripening  of 
cream,  128. 

Buttermilk,  germs  in,  122;  as  a  starter, 
132. 

Butyric  acid  ferment  in  milk,  63. 

By-products  of  milk  and  cream:  meth- 
ods of  preserving,  125;  treatment 
of,  123. 


[186] 


Index. 


187 


Carbolic  acid,  88. 

Casein  test  in  milk,  173. 

Cell  arrangement  of  bacteria,  3. 

Cheese:  bacterial  flora  of,  155;  bitter, 
177;  black  180;  blue,  180;  Brie,  164; 
Cheddar,  165;  Edam,  171,  180;  Em- 
menthaler,  162,  167;  flavor  of,  181; 
gassy  fermentations  in,  166;  Gor- 
gonzola,  162;  green,  152;  indefinite 
faults  in  cheddar,  165;  molds  of, 
180;  "nissler,"  167,170;  poisonous, 
181;  putrid,  179;  relation  of  bacteria 
to,  148,  150;  ripening  of  moldy,  162; 
ripening  of  soft,  163;  Roquefort, 
162;  Stilton,  162,  163;  susceptibility 
of,  164;  Swiss,  170,  177. 
Making  and  curing:  bacteria  in  cur- 
ing, 157;  chemical  changes  in  cur- 
ing, 154;  details  of,  149;  physical 
changes  in  curing,  153;  prevention 
of  defects,  182;  principles  of  manu- 
facture, 148;  starters  in,  151;  tem- 
perature in  relation  to  bacterial  in- 
fluence, 157. 

Chemical  changes  in  cheese-ripening, 
154. 

Chemical  disinfectants  in  milk:  bleach- 
ing powder,  73;  corrosive  subli- 
mate, 74;  formalin,  73;  sulfur,  73; 
whitewash,  74; 

Chemical  preservatives,  88. 

Children,  milk  for,  97. 

Cholera  germs  in  milk,  83. 

Classification  of,  bacteria,  4;  of  fer- 
mentations in  milk,  54. 

Coccus,  definition  of,  2. 

Cold,  influence  on  bacteria,  9. 

Color  fermentations  in  milk,  68,  70. 

Conditions  for  bacterial  growth,  5. 

Contamination  of  milk,  25;  through 
disease  germs,  75. 

Cream,  bacterial  changes  in,  120;  me- 
chanical causes  for  bacteria  in,  118, 
128;  pasteurized,  98. 
Ripening  of,  128;  advantage  of  pure 
cultures  in,  137;  age  of  starters,  141; 
artificial  starters  in,  132;  butter- 
milk, 132;  by  natural  starters,  131; 
characteristics  of  pure  cultures  in, 
135;  natural,  131;  objections  to  pure 
cultures  in,  138;  principles  ol  pure 


cultures  in,  132;  propagation  of 
pure  cultures,  138;  home-made 
starters  in,  137;  sour  milk  in,  132; 
whey  in,  132. 

Creamery  methods:  centrifugal,  121; 
modern  gravity,  121;  primitive 
gravity,  120. 

Culture  media,  18. 

Culture  methods,  17. 

Dairy  utensils  a  source  of  contamina- 
tion, 26,  27. 

Defects  in  cheese,  prevention  of,  182. 

Desiccation,  influence  of,  on  bacteria, 9. 

Development,  conditions  favoring  bac- 
terial, 3. 

Diphtheria,  84. 

Disease  germs  in  milk:  action  of  heat 
on,  81;  effect  of  pasteurization  on, 
116. 

Disinfectants:  carbolic  acid,  74;  chlo- 
ride of  lime,  73;  corrosive  subli- 
mate, 74;  formalin,  73;  sulfur,  73; 
vitriol  salts,  74;  whitewash,  74. 

Disinfectants  in  milk:  alkaline  salts, 
88;  boracic  acid,  88;  physical 
agents,  90;  formalin,  89;  preserva- 
line,  89;  salicylic  acid,  88. 

Distribution  of  bacteria  in  air,  12;  soil, 
12;  water,  12. 

Domestic  apparatus  in  pasteurization, 
106. 

Edam  cheese,  171,  180. 

Emmenthaler  cheese,  162,  167. 

Endospores,  4. 

Factories,  source  of  milk  contamina- 
tion in,  46. 

Factory  by-products,  121;  treatment 
of,  123;  preservation  of,  125. 

Fermentation,  14;  of  organic  matter,  14. 
In  cheese:  affecting  flavor,  161,  162, 

165;  gassy,  166. 

In  milk:  alcoholic,  62;  bitter,  64;  blue, 
69;  butyric,  63;  classification  of,  54; 
digesting,  65;  slimy,  61;  filmjolk, 
62;  gaseous,  59;  kephir,  63;  ku- 
miss, 63;  lactic  acid;  56;  lange-wei, 
62;  red,  68;  ropy,  63;  slimy,  63; 
soapy,  67;  souring,  56;  sweet  cur- 
dling, 65;  taettemjolk,  62;  treat- 
ment of,  70;  yellow,  70. 


188 


Index. 


Tests  173;  apparatus  for,  174;  casein 
or  rennet,  173;  Wisconsin  curd, 
174. 

Ferments:  organized,  15;  unorganized 
15. 

Filmjolk,  62. 

Filtration  of  milk,  90. 

Flavor  of:  butter,  129;  cheese,  161;  or- 
igin of,  130. 

Food-supply  of  bacteria,  5. 

Foot  and  mouth  disease,  81. 

Fore  milk,  30. 

Forms  of  bacteria,  2. 

Gaseous  environment,  influence  on 
bacteria,  7. 

Gaseous  fermentation:  in  cheese,  166; 
in  milk,  59,  168;  in  relation  to  liv- 
ing germs,  167;  in  Swiss  cheese, 
176. 

Germination  of  spores,  3. 

Gorgonzola  cheese,  162. 

Growth  of  bacteria,  essential  condi- 
tions for,  5. 

Heat,  influence  on  bacterial  growth,  8. 

Heated  milk:  characteristics  of,  92; 
action  towards  rennet,  93;  consist- 
ency, 92;  fermentative  changes,  93; 
flavor,  93;  hydrogen  peroxid  test 
in,  94. 

Ice  as  a  source  of  infection  in  milk,  48. 

Infection  of  milk:  air,  44,  47;  animal, 
32;  dairy  utensils,  26,  27,  47;  factor- 
ies, 46;  farm  exposures,  26;  fore 
milk,  30;  ice,  48;  milker,  34;  water, 
48. 

Isolation  of  bacteria,  methods  of,  19. 

Kephir,  63. 
Kumiss,  62. 

Lactic  acid:  fermentation  in  milk,  56; 

theory  in  cheese-curing,  159. 
Lange-wei,  62. 
Lardy  butter,  146. 
Light,  action  on  bacteria,  10. 

Matzoon,  58. 

Media,  influence  on  bacterial  growth, 

5,18. 
Methods:  of  isolation,  19;  culture,  17; 

of  cheese  manufacture,  149. 


Micrococcus  Freudenreichii,  61;  M. 
casei  amari,  178. 

Microscope,  use  of,  22. 

Milk:  a  bacterial  food  medium,  7,  24; 
anthrax  in,  82;  bacteria  in,  49,  52; 
chilling  of,  103,  cholera  germs  in, 
83;  contamination  of,  25;  diph- 
theria germs  in  84;  foot  and  mouth 
disease  in,  81;  for  children,  97;  pas- 
teurized, 97;  poisonous,  £5;  scar- 
let fever  in,  85;  selection  by 
acid  test,  100;  selection  for  pas- 
teurizing, 99;  sterile  as  secreted, 
25;  -sterilized,  97;  taints  in,  43;  tu- 
berculous germs  in,  76,  119;  ty- 
phoid fever  germs  in,  82. 
By-products  of:  butter-milk,  122; 

skim-milk,  122;  from  whey,  123. 
Contamination,  25;  from  air,  37;  from 
animal  odors,  43;  from  dairy  uten- 
sils, 26,27;  detection  of  taints,  172; 
disease  germs,  75;  distinction  be- 
tween bacterial  and  non-bacterial, 
45;  fore  milk,  30;  ice,  48;  milker, 
34;  relative  importance  of  various 
kinds,  38;  sources  of,  in  factories, 
46;  farm,  26;  temperature,  41. 

Milk  fermentations:  alcoholic,  62;  ben- 
eficent, 61;  bitter,  64;  bloody,  68; 
blue,  69;  butyric  acid,  63;  casein, 
173;  classification  of,  54;  chemical 
disinfectants  in,  72;  filmjolk,  62; 
gassy,  59,  167;  kephir,  63;  kumiss, 
62;  lactic  acid,  56;  lange-wei,  62;  red, 
68;  rennet,  173;  ropy,  60;  slimy,  60; 
soapy,  67;  sweet  curdling  65;  by 
starters,  72;  taettemjolk,  62;  tests 
for,  174;  treatment  of,  70;  yellow, 
70. 

Milk,  heated:  action  towards  rennet, 
93;  consistency,  92;  essentials  in 
pasteurization,  96;  flavor  of,  93;  fer- 
mentative changes  in,  93;  hydrogen 
peroxid  test,  94;  pasteurization,  98; 
sterilization,  94. 

Milk  preservation:  chemical  agents  in, 
88,  90;  condensation,  91;  freezing, 
90;  heat,  91;  pasteurization,  95; 
physical  agents,  90;  sterilization, 
94. 

Milk-sugar  as  bacterial  food,  24. 


Index. 


189 


Moisture,  influence  on  bacteria,  7. 
Mold,  in  butter,  147;  in  cheese,  180. 
Motility  of  bacteria,  4. 
Mottled  butter,  144. 
Multiplication  of  bacteria,  7. 

Nature  of  bacteria,  1. 

Natural  starters  in  cream-ripening,  131. 

"  Nissler  "  cheese,  167,  170. 

Odor  from  animals  in  milk,  43. 
Oily  butter,  146. 

Parasitic  bacteria,  13. 
Pasteurization  of  milk:  acid  test  in, 
100;  bacteriological  study  of,  116; 
chilling  in,  103;  compared  with 
sterilization,  97;  details  of,  98;  tor 
butter,  134;  in  relation  to  tubercle 
bacillus,  76;  selection  of  milk  for, 
99;  temperature  and  time  limit  in, 
102. 

Pasteurizing  apparatus:  continuous 
flow,  108;  coolers,  114;  DeLaval,  110; 
domestic,  106;  Hochmuth's,  110; 
intermittent  flow.  111;  Pott's,  113; 
Reid,  135;  Russell,  112;  utensils, 
105. 

Penicillium  glaucum,  153,  162. 
Physical  changes  in  cheese-ripening, 

153. 

Physiology    of     bacteria:    action    of 
chemical  substances,   10;   cold,  9; 
conditions  of  growth,5;  gaseous  en- 
vironment,  7;    heat,   8;    light,    10; 
moisture,  7;  physical  forces,  8;  rate 
of  growth,  7;  temperature,  6. 
Pigment  changes  in  cheese,  179. 
Poisonous  bacteria  in  cheese,  181;   in 

milk,  85. 
Preservaline,  89. 

Preservation  of  milk:  chemical  agents, 
88;   condensing,    91;    freezing,    90; 
keeping  quality.  87;  by  pasteuriza- 
tion, 95;  physical  agents,  90;  ster- 
ilization, 94. 
Prevention  methods  in  milk  contam- 
ination, 28,  35. 
Pure  cultures,  21. 

Pure  culture  starters:  advantages  of 
137;  characteristics  of,  135;  home- 
made cultures  compared  with,  137 


objections  to  use  of,  138;  propaga- 
tion of,  138;  time  of  propagation, 
141. 
Putrid  cheese,  178;  butter,  145. 

Rancidity  in  butter,  143.  ^ 
Rate  of  growth  in  bacteria,  7. 
Red  milk,  68. 

Rennet:  action  in  heated  milk,  93;  bac- 
teria in,  152;  test  in  milk,  173.' 
Reproduction  of  bacteria,  3. 
Restoration  of  consistency  in  pasteur- 
ized cream,  98. 
Ripening  of  cheese:  moldy  cheese,  162; 

soft  cheese,  163. 

Of  cream:  artificial  starters,  132;  but- 
termilk, 132;  natural,  131;  natural 
starters,  132;  principle  of  pure  cul- 
ture starters    in,    132;     sour  milk, 
132;    whey,  132. 
Ropy  milk,  63. 
Roquefort  cheese,  162. 
Rotten  cheese,  178. 
Rusty  cheese,  179. 

Saccharontyces  gluiinis,  69. 

Salicylic  acid  in  milk,  88. 

Saprophytic  bacteria,  13. 

Scarlet  fever  in  milk,  85. 

Separator  slime,  tubercle  bacillus  in, 
119. 

Size  of  bacteria,  2. 

Skim-milk,  a  distributor  of  disease, 
125;  as  a  starter,  132. 

Slimy  milk,  63. 

Soapy  milk,  67. 

Soft  cheese,  ripening  of,  163. 

Sources  of  contamination  in  milk:  at- 
mospheric infection,  37;  dairy  uten- 
sils, 26;  dirt  from  animals,  32;  fac- 
tory cans,  27;  milker,  34. 

Souring  of  milk,  56. 

Specialization  of  bacteria,  11. 

Spirillum,  definition  of,  2. 

Starters:  in  cheese-making,  151;  in 
artificial  cream-ripening,  132;  in 
natural  cream-ripening,  131;  de- 
stroying taints,  72;  pure  cultures 
in  cream-ripening,  185,  141. 

Steam  as  a  disinfectant,  28,  30. 

Sterilization,  compared  with  pasteuri- 
zation, 97;  of  media,  17;  in  milk,  94. 


190 


Index. 


Streptococcus  Hollandicus,  62,  152. 
:Stilton  cheese,  162,  163. 
Structure  of  bacteria,  2. 
Study  of  bacteria,  17. 
Sulfur  as  a  disinfectant,  73. 
Susceptibility  of  cheese,  164. 
Sweet  curdling  milk,  65. 
-Swiss  cheese,  177;  gassy  fermentations 
in,  170. 

Taettemjolk,  62. 

Tainted  milk,  43;  detection  of,  172; 
treatment  of,  70. 

Taints  in  butter:  putrid,  145;  rancidity, 
143;  turnip  flavor,  145. 

Tallowy  butter,  146. 

Temperature:  effect  on  bacterial  de- 
velopment, 6,  8,  41;  on  milk,  41,  46, 
91;  and  time  limit  in  milk  pasteur- 
ization, 102. 

Tests  for  milk:  in  cheese-making,  174. 
176. 

Theories  in  cheese-curing:  digestive, 
158;  enzyme,  160;  lactic  acid,  159. 


Treatment  of  milk:  by-products,  120; 
factory,  46;  fermentations,  70;  tu- 
berculous, 80. 

Tubercle  bacillus:  in  milk,  76;  in  sep- 
arator slime.  119. 

Tuberculin  test,  77. 

Tuberculosis  bovine,  its  relation  to 
milk-supply,  78. 

Turnip  flavor  in  butter,  145. 

Typhoid  fever,  82. 

Tyrothrix,  158. 

Tyrotoxicon,  86, 181. 

Unripe  cheese,  151. 

Utensils,  handling  of,  in  milk  pasteur- 
ization, 106. 

Viscogen,  98. 

Water:   as   a  source  of   infection,  48; 

bacteria  in,  12. 
Whey:  a   by-product,  123;   method  of 

preserving,   125;    treatment  of,  in 

vats,  123. 
Wisconsin  curd  test,  174. 


DIVERSITY 


O-J  /  U.J 


82602 


THE  UNIVERSITY  OF  CALIFORNIA  LIBRARY 


